How to populate fields present inside a map type schema in mongoose?

Question

I have my mongoose schema as follow

const instructorSchema = new mongoose.schema({
coursesByMe: [
    {
      course: {
        type: ObjectId,
        ref: "Course",
      },
      submissions: {
        type: Map,
        of: Submission.schema,
      },
    },
  ],
  active: {
    type: Boolean,
    default: false,
  },
});

My submission schema is a simple schema

const submissionSchema = new mongoose.Schema({
  user: {
    type: ObjectId,
    ref: "User",
  },
  subFile: String,
});
module.exports = mongoose.model("Submission", submissionSchema);

I want to populate the user field but I am unable to as the type of submissions is of map type schema. Any help with this would be really grateful. Thank you.