So you would like the first argument to rollapply to change based on what user selects from Dimension drop down?
If that's the case wouldn't .arg1 need to be replaced by .arg4?
Once you do that you can create a parameter named DimensionDropDown that has the names of fields you want to display.
Then write your Dimension calculation in the form
IF DimensionDropDown = 'Some Selection from drop down'
Elseif DimensionDropDown = 'Another Selection from drop down'
So when user selects something from drop down it would return a different measure e.g. sales or profit in the mock example above.