Check out the HTML Element widget: https://marketplace.mendix.com/link/component/204843
Im not familiar with the Format String widget, but according to the documentation there should be an option called ‘Render value as HTML’ under the Data Source tab.
Docs - Format String
If that doesnt work, perhaps you could try using the HTMLSnippet but modifying the HTML in a microflow to include your attribute. The Community Commons module has a stringToFile method, it might be possible to build the HTML as a string and then store it in an HTML-file that is used in the widget.
Ill emphasize that this is mere speculation on my end though.
Best of luck!