Populate a field into a form with zoho.loginuser

Populate a field into a form with zoho.loginuser

All,

I created a form to enter data for customer engagement. One of the field is "userid" and I would like to populate this with the value of zoho.loginuser so that the user is already presented with a correct value that could not be changed.
I know there is "added user" but I have old record where the added user was a different one and that now belong to a new user and I don't know/want to change all the "added user" value for that record.

For this old record the "userid" field has been already corrected to the new one.

Regards
Riccardo