The reason I ask is because I want to know if that is seen as assisting in sin. The promo code gives the person $15 off, and gives me $15 credit in the app. The service has several halal restaurants but there also several haram chain restaurants. So if I give them the code and they order something haram, is that on me even though I didn't intend? FYI I'd be giving these codes online to strangers, not people I know IRL.
Every one is held responsible for their actions. The promo codes you are giving are not some promo codes to bars, or nightclubs, but rather to restaurants or supermarkets or whatever... where something beneficial and halal can be bought.
If you give me a box of grapes, and I make wine out of those grapes, will you be held responsible? ...No
I love explaining through examples; An accurate example can clarify everything.