Use add/remove particle to add/remove a partical from the products side of the reaction.
Each product comes as a set of three objects:
1) the main selector where you can select the product.
2) a tri-state button to let you flag the particle as "m" (missing) or "M" (NOT Mass constrained) as desired.
3) a checkbox to indicate the product decays
B (Beam Bunches): T (Extra Charged Tracks): F (Fit Type): U (unused tracks):