[DeMNiX]

better this way

use it https://dev-cs.ru/resources/939/

and like that

public plugin_init()
{
    // called once, when player pressed space button or unpressed it
	RegisterKeyPressed(IN_JUMP, "fw_PlayerJump");
}


public fw_PlayerJump(id, fwdid, bPressed)
{
    // calling when pressed
	if (bPressed)
	{
		if (!is_user_alive(id))
			return;

		if (get_entvar(id, var_flags) & FL_ONGROUND)
		{
            // your code here...
        }
    }
}