In a letter of 23 February 1937 to Hugh Gordon Porteus, T. S. Eliot wrote: "Thank you for your letter of the 22nd. I am as a matter of fact engaged this evening, and I really don’t know that the prospect of failing to hear a Russian poet read English translations of his works depresses me very much."
You'll have guessed who that Russian poet was. Porteus's letter to Eliot had said:
"The enclosed invitation was forwarded to me by Messrs Faber a few days ago with a request from a friend of mine, Mr Gleb Struve, that I should pass it on to you. It arrived in my absence on a long week-end. I’m sorry the notice is so short. Possibly you are already engaged tomorrow evening: and you might have other reasons for not accepting …
"Struve is a pleasant intelligent non-political Russian: he wrote a good book on Russian Literature which you may know. He … is a little diffident about this business. Vladimir Sirin is a very young Parisian–Russian poet, some of whose works have been translated into French and German. The next number of the Nouvelle Rev. Française will print his paper on "Le vrai et le vraisemblable"… I regard Struve as a trustworthy person, or I should not waste your time with this. If you felt adventurous tomorrow, – (You would be welcome. It is a gamble!)"
This material is all from The Letters of T. S. Eliot, Volume 8: 1936–1938, edited by Valerie Eliot and John Haffenden (Faber, 2019), which doesn't provide further information about this "Vladimir Sirin". I've seen criticism of other letters collections that delve deeply into people never actually met and events never actually attended; but a brief note about this obscure poet would surely have been worth including.