Hello Dietrich,
While I have not used the API to create a webrtc phone, we do have a guide to do this with the API. Here is the documentation on how to accomplish this.
To answer your specific question though it would be during this API call /api/v2/telephony/providers/edges/phones you can set the default user.
Hope this helps!
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Cameron Tomlin
Online Community Manager/Moderator
Genesys - Employees
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