The trick is to wrap those specific text inside and then apply background image to that
This is already addressed in the post below, check for more detail
https://discourse.webflow.com/t/custom-underline-effect-on-text/
@DuncanHamra also created a cloneable which you can check in the link below
https://preview.webflow.com/preview/custom-text-underline?utm_medium=preview_link&utm_source=showcase&utm_content=custom-text-underline&preview=457de9e352f284d6a5396d73219426b6