Step 1: To get the link of the Sign-up Page, from the Admin Dashboard click on "Configure Sign Up Page".
Step 2: In the following page, click the link "View Sign Up Page".
Step 3: You get the Sign-Up Page where you can copy the link from the Address Bar at the top.
Step 4: Login to your Shopify cart.
Step 5: From the Homepage of the cart, under "Online Store" click on "Navigation".
Step 6: Under "Navigation", click on the link saying "Footer Menu".
Step 7: Under "Footer Menu", click on "+Add Menu Item".
Step 8: Then name the Page like "Affiliate Program", paste the link the of the actual affiliate sign-up Page and click "Add".
Step 9:After adding the Footer Menu, click on "Save" at the top or "Save Menu" at the bottom.
Step 10: Reload your site and you can view the new menu added to the Footer shown below that will redirect to Sign Up Page.