Head over to your 'Nav Menu' section located at the bottom left-hand side of your dashboard under Shop menu or in your Shop Editor at the top right corner.

If you want the custom link to be a part of your main menu, press the Edit button on your Enabled Main Menu then 'Add Menu Item'.

Alternatively, you can create a new menu by pressing 'New Menu'.

When you have selected "Add Menu Item," give your link/item a Title and select 'Custom'.

Enter the Web Address (URL) in the "Link/URL:" field and then press the Save Menu Item button.

Once you have added your link, make sure you change the status to Enabled and press Save in the Nav Menu page.