how to prepare process card for gg>h2>tt~ in mssm

Hi, I want to generate events for the process "p p > h2 >t t~" using mssm. I describe it as "generate p p > h2, ( h2 > t t~ )". The h2 is produced only from b b~ and not from gluon, gluon fusion of uubar. My process card is given below. What I have to define some extra? I am using "mg5_aMC"
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set group_subprocesses Auto
set ignore_six_quark_processes False
set gauge unitary
set complex_mass_scheme False
import model mssm
define p = g u c d s b u~ c~ d~ s~ b~
define j = g u c d s b u~ c~ d~ s~ b~
define j = p
define l+ = e+ mu+
define l- = e- mu-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
generate p p > h2, ( h2 > t t~ )
output PROC_MSSM_HM

Thanks in advance
Cheers,
Muhammad