Hello, @IP,
Actually, from the Elementor settings you can make it from the widget settings see the image: https://postimg.cc/F1jnWvxD but Elementor doesn’t allow it on hover so if you want this underline on mouse hover you have to use the CSS code.
simply go to the Dashboard >> XStore >> Theme OPtions >> Theme Custom CSS >> Global and the paste the below code at the bottom of the CSS box there.
.elementor-18 .elementor-element.elementor-element-5a02de0 .elementor-heading-title:hover{text-decoration:underline !important;}
.elementor-18 .elementor-element.elementor-element-f6ccf9f:hover{text-decoration:underline !important;}
Then save settings and check back to your site after clearing the browser cache.
Regards 8Themes Team.