Any time I find the need to modify the behavior of a standard FileMaker "ad hoc" find to make it more user friendly, I usually go for an approach that has the user entering data into one or more global fields and then a script generates the needed criteria in the needed find requests in order to make this happen.
The bare bones of the script might look like this:
Enter Find Mode  ---> clear the pause check box
Set Field [YourTable::YourField ; Left ( Globals::gSearchField ) ; 2 ) & "_" & MIddle ( Globals::gSearchField ; 3 ; 9999 ) ]
Set Error Capture [on]
Perform Find 
There are other possible expressions than the one I used here for entering the criteria. I may have assumed details of the format here that do not apply to your actual situation.
For other examples of scripted finds, See: Scripted Find Examples