Possibly you could use a UDF to turn on the flow at t=0.3.
Or, probably easier, run the simulation up to a stop time of t=0.3 with the inlet gas velocity (bc_v_g) set to 0, then when it stops, set bc_v_g to the desired value, set the stop time to 3s, then do a resume:
You can also use ‘pause’ to pause the simulation and change the inlet velocity while paused.