To assign a value to the key, use the .set() method. It returns the WeakMap object, so you can chain .set() calls.

const obj1 = {},
      obj2 = {};

const weakmap = new WeakMap();
weakmap.set(obj1, 1).set(obj2, 2);
console.log(weakmap.get(obj1)); // 1
console.log(weakmap.get(obj2)); // 2