TBoneNZIt appears there's no option to add it, and I imagine it's not included since I paid ~$20 years ago for this same bottle from a local disc golf shop. If it was included I'm sure it would have been listed at the bottom of the description page.
Edit: Though the top of the description does make it a bit confusing, but I would assume it's not included unless someone from massdrop replies otherwise.
MooTaters@TBoneNZ and @MooTaters Thanks for your question. You are correct, we are not including the sports cap for this drop, sorry for the confusion. We will look in to possibly adding it to future drops though.
Thanks!