The dropdown name of the floor you're on is determined by a value in the map header, which you can view by pressing Ctrl+H in Advance Map's header tab. Here's a quick and dirty rundown on what the values mean:
The byte that you need to change if you want to add/ remove/ edit the floor number is byte 27, the second last byte in the header. For most maps the byte is set to 00 which disables the floor feature. Setting the byte to 01 will say you're on F1, 02 is F2, 03 is F3 etc. This pattern ends at 63 (F99) as the game is unable to display a three-digit number as a floor number. You can also use basement floors like in Team Rocket's Celedon hideout by setting the byte to FF which will say you're on B1F, FE will show B2F, FD will show B3F and so on. This continues until 9D (B99F) as once again, the game can't display a three-digit number here.