Web components are HTML elements. They have properties and attributes. When an attribute can not be expressed with a Boolean, Number or String we need to set those values on the web component's property after we have a reference to it.
In this example I am setting the component's ueserData property so we can see how the component responds.
Notice that in order to set the component's property I had to first wait for the customElements.whenDefined promise to resolve. When composing components this is not the case. See the example here
Code is here