I'm not sure what are you talking about, but what I understood, I would do like this: (FM version ofc)
Code:
new Float:velocity[3];
pev(index, pev_velocity, velocity);
velocity[2] = 300.0; //How high you want that player gets...
set_pev(index, pev_velocity, velocity);
If you want to make it usable when player is already in air with speed, you should use:
Code:
velocity[2] += 300.0;