@T_Sidany - The example that @alex.hong was showing is using a specific List’s Subscribe and Preference page. I know it sounds a bit confusing, but each (and every List) in Klaviyo has its own Subscribe and Preference page settings.
The one you’re referencing, is the General Subscribe and Preference settings (from the Klaviyo Admin sidebar) - and this is used when a user receives an email when they are sent via a Segment (since a Segment isn’t an exact List, or a user can be part of multiple Lists) or from a Flow (since a Flow Message isn’t necessarily confined to people from a single List, or a user can be part of multiple Lists).
From there, you can grab the URL where to change/set your custom Forms’ Action value. Remember, if you are using this method, when someone subscribes in those form, you have to specify and designate the email address to go a specific List (typically, your Main newsletter List).
The guide that Alex references should walk you through the steps to do this:
Hope that clears it up a bit, and @alex.hong, feel free to chime if I missed anything!