When buying and selling Ripple (XRP), on top of the wallet address, you also have to enter a Destination Tag. This unique reference number, also known as secret key, was generated when you or your exchange created the wallet.
It's important that you put the correct destination tag when placing an order for XRP, or your coins may be lost. Here's how you can find yours, depending on which scenario suits you best:
Due to how Ripple works, exchanges typically only have a few Ripple wallets that all transactions go through. Each user gets assigned a destination tag - all transactions using this tag correlate to a specific account.
To find yours, go to your exchange or wallet provider and look for a 6-10 digit number called Destination Tag, which is the number you'll need to enter during the order process.
If you created the wallet yourself and only you can access it, you should receive the transaction regardless of the chosen destination tag. If you are absolutely sure that you don't have a destination tag, put 12345678.