How to Unmarshal jSON with dynamic key which can't be captured as a `json` in struct: GOlang

I have this struct defined:

type X struct {
 A string `json:"a_known_string"`
 B string `json:"b_known_string"`
}

This sample JSON:

jsnStr := [read in from a file and printed out to confirm]

It is:

{
 "any string" : {
   "a_known_string" : "some value",
   "b_known_string" : "another value" 
 }
}

If it was just the struct, I could:

var x X
err := json.Unmarshal(jsnStr, &x)

But I need to capture that 'any string'.
How do I do that please?

Use a map:

var m map[string]X
err := json.Unmarshal([]byte(jsnStr), &m)

playground example