What is it?
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:
The wallet was created on an exchange
Due to how Ripple works, exchanges typically only have a few Ripple wallets that all transactions go through. On top of this, each user gets assigned a destination tag - all transactions using this tag will be sent to their account specifically.
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 on the payment form in this field:
You created the wallet yourself
If you created the wallet yourself and only you can access it, you will receive the transaction regardless of the chosen destination tag.
You don't have a destination tag
The destination tag is a mandatory field, but in some rare cases, you may not have one. If you are absolutely sure that you don't have a destination tag, put 12345678.
Be aware though: if you put the wrong tag, and don't have sole ownership of the wallet, the coins may be lost.