Altering Data Source Text At Print Time With Data Entry Form?

I have a template connected to a data source that loads into the template a string of letters and numbers that is static and unchanging, followed by "XXXXXX-YYY". I want to use a data entry form with a text input and number input control that activates at print time to replace the X's and Y's with a serial number and serialization/quantity number. Ideally, the operator would type in a 6 digit serial number in the text input control to replace the X's and then specify a quantity number in the number input control to replace the Y's with 001 and then increment/serialize by 1 up to the quantity specified.

So far I can't figure out how to do this. If I link the data entry form controls to the data source field, it just loads the data source text into the text input box and won't let me change it. What do I need to link the data entry form controls to in order to achieve this?