"Do we give a tip?"
One does not "give" a tip in english. Correct english is to "leave" a tip. Or, tip can be the verb: "Do we tip?"
If someone said this to me, it would almost sound like "give a flip" this is not what you want to convey.
It's like "do we or do we not?". You could also say 我们给小时费吗？ Im not chinese enough to know if there is any real difference.
I found myself typing:
It marked it wrong but unfortunately that might just mean that duo hasn’t included 必须 in its vocabulary.
So, can I say 必须给 (as a phrase)? And does it mean what I think it means?
It depends on how you want Duo to be precise on the meaning. 必須means Must. Certainly the meaning is more or less the same if we say Must we give a tip? but you certainly know these 2 sentences are not perfect equivalent.