Create request number for excel form

Dears,
I need to create a number or alphanumberic reuwst number for the attached travel form. this number would be used on the SAP in =order to trach the reuest.
This sheet will be distributed among employees so what i need is that the code generated is not changed when the sheet is closed or saved, so that they can go back to it if they need ti track m can there be for example a button to generate the number and that the number is not changed unless the button is pressed.

Please help.

Thanks,