So, it's not a scam.

You're just not completed the document they're asking, It's on their term & condition and you cannot blame on them as well. Becasue it's part of their service rules, proof-of-source fund is always tricky for people like you.

Still cannot find the alternative for this one, I only got alternative for proof-of-address if people are still living on their parent and all the bill under other name not us.