How I can write the value of mongo objectid in json string in golang

The objectid is always unique and unpredictable. I want that, this var should take value of primitive.NewObjectID.Hex()

var mockData = `{"status":201,"message":"User Created Successfully!","data":{"data":"{"InsertedID":primitive.NewObjectID().Hex()}"}}`

But writing it like this gives me a straight string in output.

You have to "break" the string literal and concatenate the parts using +:

var mockData = `{"status":201,"message":"User Created Successfully!","data":{"data":{"InsertedID":"` + primitive.NewObjectID().Hex() + `"}}}`

This will work because the hex object ID does not need any special escaping in JSON, but in general you should use the encoding/json package to generate valid JSON (which knows about proper escaping).

Using the encoding/json package this is how it could look like:

var s struct {
	Status  int    `json:"status"`
	Message string `json:"message"`
	Data    struct {
		Data struct {
			InsertedID string
		} `json:"data"`
	} `json:"data"`
}
s.Status = 201
s.Message = "User Created Successfully!"
s.Data.Data.InsertedID = primitive.NewObjectID().Hex()

out, err := json.Marshal(s)
if err != nil {
	panic(err)
}
fmt.Println(string(out))

Try the examples on the Go Playground.