I want to use the autofill feature to complete column B (which is currently
null) so I can then concatenate Columns A&B to make unique values (see
result).

Column A Column B Result
C100 A C100A
C100 B C100B
C100 C C100C
C203 A C203A
C203 B C203B
C304 A C304A
C304 B C304B
C304 C C304C

Is anyone able to assist?
Thanks.