You do not have to specifically own Bitcoins on your CoinDCX account to receive more Bitcoins. To receive Bitcoins, click on Receive Crypto under Investments. The list of all 14 tokens on CoinDCX appears on the screen.
Click on Bitcoin and please make sure you do not select any other crypto. This is because Bitcoins sent to other wallets might get lost forever. Once you have ensured that you have selected the correct wallet (Bitcoin in our case), click on Yes, I Understand.
You can either share your public address or the QR code. Make sure the public address you share is the correct one as Bitcoins once transferred will not be recovered.