apply formula on the default text in interface tool

[PangHC]

for example, the default filename is

FileA_P10.xlsx the P10 mean the month. and i want to give a freedom to user to amend or change the filename.  

hence, i put the interface text box to show the default text built by formula, but it only able to input text only.

from this > "FileA_P10.xlsx"

to this   >   "FileA_P" + datetimeformat(datetimetoday,"%m") + ".xlsx"

and it is better if it apply for other interface tool.