What do you mean by that (“custom html tag”)? If you want to add a new attribute to an element, you first need its ID. Once you have that you can do whatever you need in the page vis-a-vis attributes.
Are you referring to actual tags? Like, heading tags (h1, h2, etc.)? If so, you can select those properties to use either in the inspector for text elements:
… or (if the text element is using a style, the HTML tag setting migrates into the style itself), in the Style properties for a given Style (Styles tab):
Or are you asking about something else?