I don't know if there it's coded something for dismount the bike.
For check if you're in the bike, I used this:
special2 LASTRESULT 0xE3
compare LASTRESULT 0x2
if 0x1 call 0x(MACH BIKE)
compare LASTRESULT 0x0
if 0x1 jump 0x(BY FOOT)
#org 0x(MACH BIKE)
msgbox '"¡Comienza la carrera..."
#org 0x(BY FOOT)
msgbox '"¡No te acerques al b..."
This is tested in ruby, I suppose you're asking for FR, but well, try it.
Maybe you'll need to 'compare' the 0x1 for be in the FR bike, like in FR there is only one.