[Champ]

don't know about left alt but you can choose any of the keys from the list in the link below:
https://sampwiki.blast.hk/wiki/Keys

I am using key crouch.

pawn Код:

#define PRESSED(%0) \
    (((newkeys & (%0)) == (%0)) && ((oldkeys & (%0)) != (%0)))

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys)
{
    if (PRESSED( KEY_CROUCH ))
    {
        new FLOAT:x, FLOAT:y, FLOAT:z;
        GetPlayerPos(playerid, x, y, z);
        SetPlayerPos(playerid, x, y, z+25);
    }

    return 1;
}

this may fulfill your needs. If you want to make a jump higher then just increase the value after flaot Z.