The easy way would be to add a regular button labeled Login and link it to any members only page that requires a pricing plan. Pop up will come up on its own to prompt them to subscribe to a pricing plan.
The intuitive way would be to code a button to prompt login, check who is logged in, if they have a pricing plan allow sign in to continue, if no pricing plan redirect them to the pricing plan page.
Or you also have the option to do nothing. Simply install the login bar and mark every single members only page via the settings, with the free pricing plan or other plan. As soon as they attempt to view the page Wix pricing plan will prompt a message asking them to select a compatible pricing plan to view that specific page. <——— A lot of people like this option better so that they can promote member traffic on their site while natively allowing the member to upgrade to another plan they don’t already have. Hence, upselling.