Hello,
I am not familiar with Link Mobility API and its Authentication/Authorization process.
But I think you may be confusing what the Outbound Notification Webhook Signature Secret Token is. It is not an API Key equivalent - I mean it is not sending the secret token in an HTTP Header. Its purpose is not about authorization (like an API Key sent via a specific HTTP Header name).
Each webhook notification with its message will contain a signature (sent through X-Hub-Signature-256 header). Along with the secret token that you have set in the Open Messaging configuration, it will allow to validate the message with its signature. You can find a code description here: https://developer.genesys.cloud/commdigital/digital/openmessaging/validate#validate-a-notification-from-an-outbound-open-message
So you'd likely need a gateway (API gateway or equivalent) to extract info from the webhook notification and propagate this to Link Mobility adding the necessary authorization headers.
Regards,
------------------------------
Jerome Saint-Marc
Senior Development Support Engineer
------------------------------