[Plugin: WP Maintenance Mode] Where do I put "Own Theme" custom HTML?

For WP Maintenance Mode plugin … I’d like to use my “Own Theme” but where do I put my custom html file and how do I direct the plugin to open that custom page? The url field in the plugin settings only mentions about pointing to the css file …

Also, does the html file need to be named anything specific or whatever I want?

Thank you,
Ben