Hello T2, welcome to English Forums! Yes, 'whoever' is correct. It is the subject of the verb 'is designated', so it has to be 'whoever'.
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Or simply,
"To whomsoever it may concern"
MrP
"To whomever it may concern"
AnonymousThanks a lot for the test you suggested.Phew! And so few things do.
It helps,it actually does.
Huuuuuuuuurrrrrrrrrrrrraaaaaaaaaaaaaahhh !!