line breaks in form element input value

The HTML input elements, type text or something similar (such as email) will deliberately strip off all line breaks, so that’s not going to work.

The only form element which will accept line breaks is the textarea.

Incidentally, Clipboard.js works by creating a dummy textarea element to do its magic.

I don't actually think you can have line breaks for those types of form elements. You may have to use a <textarea> tag which allows for literal line breaks:

<textarea id="multiliner" name="multiliner">line1
line2
line3</textarea>