Is it possible to add a weapon proficiency to a spell? Basically I have created a cantrip similar to Booming Blade or Green-flame Blade that requires a melee weapon attack, but it only works with whips. I want the spell to be able to grant them proficiency with that weapon, but ONLY when using that spell. I have the spell created and such and all of the damage numbers calculate fine for a martial weapons character. For a non martial (sorcerer) it treats the whip as non proficient. All of the spell damage calculations are correct on the item tooltip, however. I mean the player could manually add their proficiency bonus to it, I was just wondering if there was a way to get the char sheet to display it. 

I attempted to do this by adding a modifier field for Profciency and Whips, but I left the Dice drop downs blank because it seems like those are to add static numbers?

The spell won’t actually put that proficiency on the character sheet. At least not yet. You can make the spell, but the target will have to add the proficiency manually, and then remember to remove it when the spell ends.