How do I send Unique ID's and their Names from Another Platform to the Form?

I would like to know if it's possible to do a GET on another Restful platform (json) to get user first and last name along with their backend ID and populate them in a dropdown in a form or a lookup table. The other platform uses OAuth 2.0 to authenticate. Then when the form is completed Zapier sends the from data with the userID and updates the platform instance (POST). 

It is possible to populate the data via the Form URL into the Form. You could get the data somehow programmatically via your website to the form.

This guide would help you with achieving this:

https://www.jotform.com/help/71-Prepopulating-Fields-to-Your-JotForm-via-URL-Parameters

Demo Form:

https://form.jotform.com/200891285521050?populatedData=Hi this is a test

Once the Form is submitted you can trigger the Zap to insert the data to your end.

Let us know if you have further questions.

Thanks.