Connecting Postscript is a two-part process: you connect the account once in Settings, then use it inside a Playbook.
Part 1: Connect your account (one-time setup)
1. In OuterSignal, go to Settings → Integrations → Postscript and select Connect.
2. A window opens asking for your Postscript API key.
3. Log in to Postscript (in a separate tab) and select the gear icon in the left-hand menu to open Settings and select the API tab.
4. Select Create Security Key Pair (don't use the legacy API key — it won't work with this integration).
5. Name your API key and copy your Public Key.
6. Back in OuterSignal, paste the Public Key into the Postscript connect window, then select Connect Postscript.
7. The integration card flips to Connected.
Note: Postscript's API is plan-gated. If you are on on a Postscript plan without API access, your key will still look valid and connect successfully — it just won't work later. If properties aren't showing up in Postscript after connecting, check your Postscript plan first.
Part 2: Sync data to Postscript
In OuterSignal, go to Playbooks → Create Playbook and pick the segment you want to act on (use All Customers to apply to all OuterSignal Customers).
Choose Postscript Subscriber (listed under the Postscript integration card).
Under Custom Properties, map each property name to an OuterSignal value — for example, outersignal_persona → {{persona}}, or outersignal_segment → {{segment.name}}. You can map up to 50 properties per subscriber (Postscript's limit).
Choose an Update Mode — Patch or Put (see below).
Select Add Action → Review & Create → Create Playbook.
Patch vs. Put
Patch (merge properties) — recommended, and the default. Only the properties you've configured are written, and only when there's a value for that customer. Everything else on their Postscript record — other properties, email, tags — is left exactly as it was. Nothing is ever removed.
Put (replace properties) — behaves the same as Patch, with one difference: if one of your configured properties has no value for that customer, it's removed from their record instead of left in place. Useful for cleanup when someone leaves a segment.
Put only takes effect on Playbooks triggered by a customer exiting the segment. On enter or while-in-segment triggers, the action always behaves as Patch, regardless of which mode is selected — this is deliberate, since clearing properties out mid-membership isn't usually what you want.
Either way, the action never touches the subscriber's phone number, opt-in status, email, tags, or any properties you haven't configured.
A few things this action doesn't do
It only updates customers who are already Postscript subscribers. OuterSignal never creates a new subscriber, since doing so would trigger a real opt-in SMS to that customer.
Matching is by phone number, and it has to be in international format (e.g. +15551234567). If a customer has no phone number, a badly formatted one, or isn't a Postscript subscriber yet, they're quietly skipped — nothing is written for them.
Part 3: Finding and using your properties in Postscript
You don't need to create the custom property ahead of time — the first time a Playbook writes to it, Postscript creates the property automatically if it doesn't already exist.
To confirm a property is receiving data:
Log in to Postscript and go to Subscribers.
Open a subscriber who should have synced.
Check their profile for the Custom Properties section — any property set by the Playbook will show there with its current value.
How to use a property:
In an automation filter: in the Flow builder, add a filter step, select Custom Field, choose your property name (e.g. outersignal_persona), then set the condition (equals, contains, etc.).
In a segment: go to Subscribers → Segments → Create Segment, and build a filter the same way — select the custom property and the value you're targeting.
One thing worth knowing: since Patch mode (the default) never removes a property, once a subscriber has a value set it'll stay there — even if they later leave the segment that originally set it — unless you're using Put with an exit trigger (see Part 2).
FAQ
Where do I enter my API key? Always in Settings → Integrations → Postscript — never inside a Playbook.
Do I need anything besides my API key to connect? No — just the Security Key Pair from Postscript's API settings, plus an optional display name. OuterSignal doesn't ask for a shop token or partner key.
I connected Postscript but don't see any properties showing up. What's wrong? Two likely causes: either no Playbook has the Postscript Subscriber action added yet (connecting alone doesn't sync anything), or the account is on a Postscript plan without API access — check that first if a Playbook is already set up.
Which update mode should I use? Patch, unless you specifically want stale property values cleared out when someone drops out of a segment — then use Put with an exit trigger.
Why didn't a specific customer's properties update? Most often it's because they aren't a Postscript subscriber yet, or their phone number is missing or not in international format (+15551234567). The action skips them silently rather than erroring.