Email with button to dynamic URL from Bubble

So i have managed to configure it so I can send emails from bubble through sendgrid and it brings up the template!
however, Im a little confused to how sendgrid works. Ive been reading up on their API and so within bubble - I have this as my email setup

On Sendgrid, I have this as the template:

When I trigger the email, the test image appears, %body% becomes testworld, which is great, but how do I send reference the button to cause an action (ideally as a navaigation back to bubble which will also alter a field for the user allowing me to trigger a new workflow?)
I cant seem to find it anywhere in their docs :confused:

thanks so much - Alex