How to reference a registration screen field.

Hello. 
In a registration form, I have a field that requests a membership number and once I validate that it exists (through AJAX),
I show an ok message.
I would like to fill in a text field with the name of said partner, but I don't know how to reference the screen field, since it has not yet been saved in the Database.
The query I use would be this:
SELECT CONCAT (firstname, ' ', lastname)  FROM #__comprofiler 
WHERE cb_num_socio = [cb_num_socio]

Thanks

I assume you're using CB Query Field and its Query Validation for this? It just validates. It has no functionality to fill out fields.

Can you better describe what you're trying to do exactly and where? Is this just meant to display on profile? If so I'd just use a Query Field and you can query for that information as follows.

Query:
The above should work during registration as well if you combine it with CB Core Fields Ajax and its "Update On" feature. You'd have it "Update On" the cb_num_socio field so its display refreshes during profile edit and registration when that fields value changes.

If you need it stored in the database you have a couple of different options. You could use CB Auto Actions and a Query action to push that information into the database.

You could also use something like Query Auto Complete on a Text field to give a dropdown selection of names matching that field. Example as follows.

Integrations > CB Query Field > Auto Complete
Query Auto Complete: Enable
Query:

OK, issue resolved.
Sorry for the lack of details when explaining the problem, but you understood me perfectly.
I have included the field with CB Core Fields Ajax and its "Update On" and it has worked correctly.
 

Thanks.