How To Get Your Steam URL
Step 1: Go to your Steam profile by hovering over your Steam name and click 'profile'. This can be done on the Steam website or in the application.
Step 2: Right click on your Steam profile page and then click 'Copy Page URL'.
Step 3: Paste your Steam URL where it's required and click add to cart. Don't worry if you have a custom URL, it can be converted to your 64 ID.