Will a referendum on leaving the Russian Federation be held in the Russian region of Belgorod? No, that's not true: Belgorod's Election Commission denied a referendum was planned and Russia's Federal Constitutional Law expressly forbids referendums to change the status of the Russian Federation.
The claim appeared in a 5 May, 2023 TikTok video (archived here) under the title "Referendum in Belgorod is ready".
Below is how the post appeared on TikTok at the time of this writing :
(Source: TikTok.com screenshot taken Sun May 14 9:35:25 2023 UTC)
The Belgorod election commission issued a denial on the city's website saying no such referendum to separate the region from Russia was planned and that the purported ballots shown in the video had telltale wording errors.
According to paragraph 5.1 of Article 6 of the Federal Constitutional Law from June 28, 2004, On the Referendum of the Russian Federation:
questions about changing the status of the subjects (regions) of the Russian Federation, enshrined in the Constitution of the Russian Federation, cannot be submitted to a referendum.