Add or Edit Record

OK So i have 2 tables, one for user auth, one for the user data, I have a link as users_id on the user data table. I use the auth table that is connected to the sign up form on the front end to sign a user up.

Once a user signs up the user sign up info is saved to the users table. Now the user has to add information, that is done through the user data table. On the new table I have an API Post that will add or edit the record from the front end. The problem is I can never the the 'Add' part to work. I get a vauge error Unsupported parameter reference - xdo.5 which i found out is reference to the pre-populated ID which obviously doesn't exist because it needs to be created. Why then is the 'Add' part not working and how can I better set this link up so it doesn't keep erroring. Is there a better way to link databases and do a lookup?
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Add or Edit Record - XDO Error