How to underline the first line of Textarea?

[Clarity]

Hello everyone!

I have a text in Textarea and need to underline the first line of it (append class="u-bottom-underline"). By the line I mean the part of Textarea before the first "\n".

I've checked the topic:

 

But it's not quite what I need.

Thank you in advance!

[3fingers]

I think the best approach would be to use css ::first-line pseudo selector for that.