Please follow these instructions to show/hide the custom design tab:
Navigate to the admin of Sign Customiser on your store
Click on "Customisers" in the home screen menu
Click on the customiser you wish to edit
Click on "Custom Design" in the navigation menu
Click on "Link to custom design page"
Scroll down to the "Custom design link" card and make sure "Custom design screen with additional instructions" is checked.
Fill in the "Custom design link" input with the full URL you want the button to redirect to when clicked.
Hit save at the bottom of the form.
Here's what the Custom Design tab looks like when it's set to Hide:
Here's what the Custom Design tab looks like when it's set to Show:
