Did you set the Performance settings in the menu, and then the Takeoff Parameters? WT21 needs to know FL cruise, OAT, Wind Str/Dir, local altimeter and RWY/SID. Then it will generate the three ref speeds for T/O and then you can “send” it to the FMS for appearance on the tape.
I haven’t tried the mod myself yet, IRL is hitting hard today, but if Husk kept most of the WT21 code, that’s how that works.
Recommend reading briefly through the Operator Gude written by WT. You’ll pick it up real quick.