I want to add a checkbox to Learndash checkout page

I a php and learndash beginner and need help. I would like to add a checkbox to the checkout page in learndash.

Can you have a hint which action hook will do. The checkbox should be added to the order overview page This page is shown after registration and has a buynow-button.

enter image description hereI have tried a lot of hooks without success.

Thanks a lot for your help