You can add a contact link to your checkout page by following the steps below:
Step 1: Click on the 'Edit' button to edit your checkout page in Customizer
You'll see the Customizer open up.
Step 2: Click on the Header widget
Step 3: Scroll down to the 'Contact Information' section
Here you can edit the Helpdesk Label or Helpdesk URL.
Step 4: Click on the 'Publish' button to save your changesOn clicking 'Publish' all your changes will be saved.
So when you preview your checkout page and click on the 'Contact Us'/'Helpdesk' button, here' the output you'll get: