ID number generation - with prefix

Hi everyone,

I am currently working on a personal project, creating a macro in excel to better manage a register of businesses.
It is incomplete at the moment.

Currently, I have a userform with a field licencetext where the user can enter a number which acts as an ID number (the prefix "FB" is added during transfer of data from the userform to the sheet.

However, if the field is left blank, a would like excel to generate the next unique three digit number available (and verify that is not a duplicate)

Any ideas on how to do this? I have had a look at some other threads on the net, but can't seem to find something relevant or easy to understand - I am new to VBA and entirely self-taught mainly through just google and looking at examples... yea I know, it's pretty bad form XD


Edit. Attachment included - please note the code and layout of userform are not complete