I can't figure out how to do this - my webcoding knowledge is too limited

I have a customer support phonelog, to record phonecalls and cases from customers. We are several co-workers who's doing this support task and each has their own login.
In the subform (logging of customer calls) I was hoping to find a way for the column co-worker (labeled 'Medarb') to be automatically chosen (filled in) based on the logged in users ID.
I have a table for co-workers ('medarbeider') which has a connection to phonelog table ('telefonlogg') with a foreign key in phonelog table ('tel_medID') pointing at the primary key in table 'medarbeider'.
In the table 'medarbeider' (co-workers) I also have a column containing a copy of each users nuuserID (for instance '5ce170db2ae06e3').
Is it possible to have the subform phonelog ('telefonlogg') to fill in the column labeled 'Medarb' automatically when saving - if (!) the column is empty?
