index signature and typed key in TS?

The following code will always produce an error, but as a sketch, this is what I aim to represent:

interface A  {
    [ key : string ] : string, 
    "anotherKey" : MyInterface
}

I’ve checked a few books, TS documentation, tested in ts-node, but I am not that well versed and need some help.

How can I specify an object whose most keys are "key":"value" but some specific aren’t?

>Solution :

Use a regular type and intersection:

type MySpecialType = {
    specific: { type: number; };
    extremely: { specific: { type: string; } };
} & {
    [key: string]: string;
};

& is read as and

So this is saying this is a really specific type and an index signature of strings to strings.