Step 1: Locate the Main menu in storefront
From the Shopify admin page, click to Online stores>Navigation
From the Navigation page, click Main menu
Step 2: Add the form to the menu in storefront
Once you are in Main menu, click on Add menu item
Add menu item popup will open in right hand side
Fill the name and add the link by punching in the Registration Form Setup. You can copy your Registration form link from here (Dashboard > Registration Form Setup). Now You can view the Registration Form in the Frontend menu section.