I Think It's, Technically, Illegal.

https://www.rt.com/russia/552510-negotiations-russia-scandalous-zelensky/

Ukraine holding a referendum on the status of Crimea along with the Donetsk & Lugansk People's Republics.

Who have both already exerted their Right of Self-Determination under the Montevideo Convention/UN Charter.

It would be akin to Britain holding a referendum on making France part of Britain.

When Britain won't hold a national referendum on the status of Scotland.