E.g. copying the disclaimer from my staff page https://www.mediawiki.org/wiki/User:Jdforrester_%28WMF%29 takes:

<p><b>Disclaimer:</b> Although I work for the Wikimedia Foundation, contributions under this account do not necessarily represent the actions or views of the Foundation unless expressly stated otherwise. For example, edits to articles or uploads of other media are done in my individual, personal capacity unless otherwise stated.</p>

… and turns it into the wikitext:

'''Disclaimer:'''
<nowiki> </nowiki>Although I work for the Wikimedia Foundation, contributions under this
account do not necessarily represent the actions or views of the
Foundation unless expressly stated otherwise. For example, edits to
articles or uploads of other media are done in my individual, personal
capacity unless otherwise stated.

… (with the newlines showing before saving; not a Parsoid bug).

No new lines added when doing a plain paste.