In 1945 the USSR succeeded in obtaining full U.N. member status and a seat in the general assembly for two of its constituent republics, the Ukrainian SSR and Byelorussian SSR; a condition which was maintained until the dissolution of the USSR in 1991.
I know that a Ukrainian permanent mission to the United Nations, with its own (Ukrainian) Permanent Representative existed throughout the Soviet Era (and it's reasonable the same was so for Byelorussia) - but how were these missions run? To what degree were the U.N. representatives of these countries allowed any autonomy of action? Did the staff of these missions see themselves as representing the voice of their republics or were they essentially rubberstampers of USSR policy.
I wrote this up as a reply in response to a deleted comment asking if the two really had become members but I think it adds value enough to post as it's own sans it's parent:
. . .Nope, those two became members. In fact, although I can't speak for the Belarusian SSR, I know that the Ukrainian​ SSR wasn't simply a "background character" within the structure of the United Nations - it was actually a security council member twice 1948-1949 and 1984-1985 and sat on a bunch of important committees; Which is one of the reasons for my own present curiosity on the degree of agency it had.
It was part of the haggling around the setup of the United Nations at the end of World War II. At it's founding, there were a number of signatories that were not independent states (notably India and Philippines), as well as Dominions in the British Empire like Canada and Australia that had a somewhat ambiguous status.
Stalin's feeling was that if the British and the US got these General Assembly "votes", then the USSR should get similar subnational/nonindependent General Assembly votes.
The lore is that Stalin wanted all Soviet republics to have General Assembly seats (which would have been 16 at the time), and President Truman countered that if that happened all 48 US states should get seats too, and seats for Byelorussia and Ukraine were the compromise.
As far as how these missions were run and what independence they were given: I would have to imagine it was nil. Anyone serving in these missions would be communist party members, and be Soviet citizens. Although the non-Russian Soviet Socialist Republics had their own local Communist parties and local governments, in practice this meant very little - party bigwigs were moved from Moscow to the Republics and back all the time, and GOSPLAN (state economic planning) regions meant more in practice than republics did.
The republics didn't begin to develop their own independent outlooks and policies until very late in the Soviet period: mostly around 1990 when the republics were allowed to strengthen their governmental structures, hold semi-competitive elections, and develop their own sense of sovereignty and policies.