With maps, we use the set() method to add new values. Like the push() method used with lists, set() results in a new map:

const myMap = Map.of(
  'one', 1,
  'two', 2,
  'three', 3
);
const myChangedMap = myMap.set('four', 4);

console.log('myMap', myMap.toJS());
// -> myMap { one: 1, two: 2, three: 3 }
console.log('myChangedMap', myChangedMap.toJS());
// -> myChangedMap { one: 1, two: 2, three: 3, four: 4 }

You have to supply the new key for the map, which can be any type of value, not just strings, along with the value itself. This results in the new map being stored in myChangedMap, which has the key-value pair that you've just set.