Assign a value from a custom field to another field in the "Create a new ticket" window

I have RT 3.8.1 currently installed and running. I have some programming
skill in Perl, but am slightly weak on the HTML side. I am trying capture
the value “Selected” in a custom selection field and assign it to another
field, in this case, the “Initial priority” field. I created a custom field
as suggested in the O’Reilly book called “Severity”. I can create a simple
line of code to change the value manually, but have not been able to capture
the selected value.
The line
% $ARGS{InitialPriority} = 50;
works OK right after line 180 of the original "html/Ticket/Create.html"
I hope that this is not too trivial a question, as the answer will go a long
way in my understanding the code in general.

Mark P. Fraser