Not sure what you mean with PMO, but I take it you want it to be something like this:
Key Space is being pressed
- Set Object's Flag #0 to ON
- Add 1 to Object's Alterable Value A
Key Space is NOT being pressed
Object's Flag #0 is ON
- Set Object's Y position = Object Y - Object's Alterable Value A
- Substract 1 from Object's Alterable Value A
Alterable Value A is = 0
- Set Object's Flag #0 to OFF
Note that this one is far from perfect, but since you're not mentioning anything about having your own engine or such, this one should fit pretty well with what you're looking for. There's one flaw in this presentation which is that you can jump in the air once the "jump" value has gone to 0 (currently assuming you're using the in-built engine that does gravitation for you), but since I'm not very aware of the situation this is all I can offer at the moment.
Are you using sensors or anything like that? Backdrop objects?