Hi noppojp (this is a little late, I know).
Actually this can be done without code. First, add an extra column to your unrelated list -- you will probably have to do this in your database, and call the field in it @change (or anything else). Give it an integer datatype by doubleclicking on the field and changing the selection that says Text (string) to Whole Number (integer).Then add another field outside of the list in your datasource, called @current. Give it an integer datatype too. Then, add a button to your form, and add the following rules, in order, to it: Set a field's value: @current = <ID of Row to Get>, and Set a field's value: @change = @change + 1.
Finally, add a rule to @change: If <ID of Row> is equal to @current, THEN: Set a field's value: <Field to Update> = <Value to Change it T>.
Have a great day and I hope this helps!
Qwerty