I have not done that in MANY years, but if memory serves me it works something like
M99 (either N or P) then the line number
This is not going to be a conditional jump (with IF-THEN) nor are you going to be able to use M99 as a typical "return".
Like I said, it has been a very long time since doing that so I would suggest opening up that FANUC programming manual and look at the descriptions for both M98 and M99.
Sorry, that's all I got.