REST API - Cannot set user for mention macro added via changing page value

I am trying to edit a series of pages via the REST API by adding mentions. The user provides the names for a few key values placed on the pages. I am able to get the users’ account IDs’, but the mention macro appears to need user keys (see below.) It appears I cannot get the user key due to the data migration. Do I have any options here?

What the macro stores in the

<ac:link><ri:user ri:userkey="user key" /></ac:link>

Note: “user key” != user account id

See my comment here for working around this issue:

Thank you for the help! Works great!