In the installation contribution part of your URCap you add and remove functions that can be used to assign variables during run time. Here is an example of how to add/remove functions to the function model. 
The functions will need to be within the generateScript(ScriptWriter writer) portion of the URCap installation contribution example: 
Adding the function to the function model would then be: 
Now within the program tree of the UR you can assign a variable from the function drop down menu example:
Sorry for long post but hope this helps you out!