By default, the overlay installation of the map is triggered by clicking the launcher. If you choose to disable the launcher on your website, you can still use custom HTML links to show your map instead.
For example, you can have a link/button on a page that says "View Our Locations" and it can open the map as an overlay without ever changing the page.
Adding Overlay HTML Links
Alternatively for Shopify websites, you will have to use an href value of "#closeby-open" when creating these custom links for this to save properly.
When this link is clicked the Closeby overlay will open. Now that you have this link working you can choose to disable the default launcher by going to your map customizations unchecking the "Enable Launcher" option.