Can you add any proof?
This exchange has a 'good volume' in CMC[1]. Probably they wash trade it though. But if they did request you to send BTC before you can withdraw, I'm surprised why this exchange is not removed from CMC.
[1]
https://coinmarketcap.com/exchanges/topbtc/I dunno what it takes to get an exchange or token delisted from CMC if it turned out scam but most certainly many scam exchanges or coins appear on their list. So I don't usually use them as a standard to gauge the authenticity of the project.
For example... p2pb2b exchage has scammed so many people and i believe you have seen a number of scam accusations here but it still appears on the CMC list with a cool looking "daily trade volume"
Maybe OP should include more details like screenshots of them trying to ask for BTC. But it looks like a scam attempt to me