AvatarMask 'use skeleton from' replicate from script?

Unity Answers has lots of questions regarding AvatarMask and AvatarBuilder without any answers, so thought I'd try here.

I'm trying to replicate the process of editing a Avatar Mask in the Editor with script. I need to replicate the 'Use skeleton from' function, but can't see how to do this.

Accessing the transforms then looks easy enough. Is this possible?

 

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