So i am assuming you are showing “this form” of yours already.
Start by changing the UX which shows this form. That is, have a button which calls a Microflow.
In this microflow, first activity should be calling the API and getting the data. Hope this part is done?
Once data is there, get the needed data into a Create Object and fill the attribute values with the data that you got from the API.
Next show your form which has the context object as the same entity to which you just had the data filled.
Does this clear out the answer for you?