"Do you have stamps?"
Translation:Hast du Briefmarken?
How did you want to use sie?
There's no translation of "Do you have stamps?" that uses (lowercase) sie.
"Hast ihr"? Ugh... Alternate options that are not multiple correct answers again...
All three answers were the same Hast du Briefmarken, I selected one and it said I was wrong.
Can you provide a screenshot?
Check that the distractors were not non-words such as Briefemarken or Briefsmarken with an extra -e- or -s- in them.
How do you know if you need to add an "s" when combining words? Is there a general rule or something?
If you can read German, then have a look at the Wikipedia article Fugenlaut: https://de.wikipedia.org/wiki/Fugenlaut
Essentially, though: it's probably best just to learn compound words as single items rather than trying to derive some rule as to whether a sound gets added between the two and if so, which one (-e-, -n-, -s-, something else).
Sometimes, there are even multiple possibilities with different meanings: a Gasthaus is not the same thing as a Gästehaus, for example.