To find available keywords or inspect keyword replies, use Get Keywords endpoint.

After adding a subscriber via this endpoint, they will receive 2 messages:

A confirmation that they are subscribed
The keyword reply

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Note

Adding the field shopify_customer_id or email_address to the metadata object will help Postscript identify which customer belongs to the phone number you are sending.

The field metadata.tags will be stored on the subscriber, so those subscribers can later be filtered into segments by these tags.

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Text Messaging Compliance
At Postscript, we take compliance very seriously (and think you should too!). What that means for our partners is that all subscribers added using this endpoint will be required to confirm they have opted in to receive recurring automated messages. If a subscriber does not confirm, then they are not added to a merchant's subscriber list.
The exact language of the confirmation message is: [Business name]: Reply Y to subscribe to recurring automated promotional msgs (e.g. cart reminders). Msg & data rates may apply.
This message is currently not customizable.
Another thing to note about compliance is that wherever you are collecting a subscriber's phone number (e.g. a form) you should make sure to include TCPA-compliant language, for example: By subscribing, you agree to receive recurring automated marketing messages at this phone number. Reply STOP to unsubscribe. Msg & data rates may apply. View our Terms of Service for details.
Finally, we've also put together a helpful guide on Text Message Compliance that you can refer to for more information on compliance best practices.