I have never used coinbase, my wallet is Electrum. A transaction id should look like this one
https://blockchain.info/tx/e93d6827e89027a70ed76184590b6dcde7784883fa9ea622ba67376e11d92e97
the text after the tx, you see. Look at the info of your transaction in coinbase, you should find something similar to
e93d6827e89027a70ed76184590b6dcde7784883fa9ea622ba67376e11d92e97.
PS:
The reason why i don't use webwallets is because most of them don't give you the private key of your addresses so they could steal all your money and go away.