Are you trying to only have the underlined as editable?
I'm sure you can combine some custom element h1 tags, wrap one of them into a span and only make that editable.
Hey Here's my Explanation & Preview Link to Copy the Code 🚀
Preview Link
Webflow Link
TLDR:
Add a span with class underline in heading, keep the span position relative, add the CSS ::after
Property to add width & position variables.
Text Shadow is optional if you want to do that line going under the text thing. since you need to adjust that in responsive viewports!