Может так хотел?
https://www.typescriptlang.org/play/index.html#src=export%20const%20enum%20MyEnum%20%7B%20first%20%3D%20'someKey'%2C%20second%20%3D%20'differentKey'%20%7D%0D%0A%0D%0Aexport%20interface%20MyInterface%20%7B%0D%0A%20%20%5BMyEnum.first%5D%3A%20boolean%3B%0D%0A%20%20%5BMyEnum.second%5D%3A%20number%3B%0D%0A%7D%0D%0A%0D%0Afunction%20someMethod%3CT%2C%20K%20extends%20keyof%20T%3E(obj%3A%20T%2C%20key%3A%20K)%20%7B%0D%0A%20%20return%20obj%5Bkey%5D%3B%0D%0A%7D%0D%0A%0D%0Aconst%20o%3A%20MyInterface%20%3D%20%7B%0D%0A%20%20%20%20someKey%3A%20true%2C%0D%0A%20%20%20%20differentKey%3A%201%2C%0D%0A%7D%3B%0D%0A%0D%0Aconst%20a%20%3D%20someMethod(o%2C%20MyEnum.first)%3B%0D%0Aconst%20b%20%3D%20someMethod(o%2C%20MyEnum.second)%3B%0D%0A