Hi Muhammad,
You need to go in the IMG (t-code spro) and find the Time sheet profiles for field selection:
When you select the option, you will be presented with different options, select the Data Entry Section by double clicking on it.
You then need to select the profile for which you want to had the field by clicking on "Influencing". Once te profile is selected, make sure you press enter for the radio buttons to be updated. You can then file the field CATS-UNIT and make sure you select "Input":
As for the user-exit, you can go through t-code CMOD and create a project. You can use CATS0002 or CATS0003 Enhancement depending of when you want the data to be set.
Regards,
Serge