Example:

const collection = new Cesium.EntityCollection();

//Create a new entity and assign a billboard scale.
const object1 = new Cesium.Entity({id:'object1'});
object1.billboard = new Cesium.BillboardGraphics();
object1.billboard.scale = new Cesium.ConstantProperty(2.0);
collection.add(object1);

//Create a second entity and reference the scale from the first one.
const object2 = new Cesium.Entity({id:'object2'});
object2.model = new Cesium.ModelGraphics();
object2.model.scale = new Cesium.ReferenceProperty(collection, 'object1', ['billboard', 'scale']);
collection.add(object2);

//Create a third object, but use the fromString helper function.
const object3 = new Cesium.Entity({id:'object3'});
object3.billboard = new Cesium.BillboardGraphics();
object3.billboard.scale = Cesium.ReferenceProperty.fromString(collection, 'object1#billboard.scale');
collection.add(object3);

//You can refer to an entity with a # or . in id and property names by escaping them.
const object4 = new Cesium.Entity({id:'#object.4'});
object4.billboard = new Cesium.BillboardGraphics();
object4.billboard.scale = new Cesium.ConstantProperty(2.0);
collection.add(object4);

const object5 = new Cesium.Entity({id:'object5'});
object5.billboard = new Cesium.BillboardGraphics();
object5.billboard.scale = Cesium.ReferenceProperty.fromString(collection, '\\#object\\.4#billboard.scale');
collection.add(object5);