Your Claim ID is located on your claim in your activity page. You can filter your activity page at the top to show claims only, or simply scroll through until you've found your claim.

Pressing the 'more' button on the right side will reveal a drop down with all the information relating to your purchase, such as the Claim ID, the purchase amount and when the claim last changed status. Your Claim ID will be visible here, as highlighted in the below image.