This is functionality that is planned for upcoming versions of 4.x, but that are not part of the 4.3 release.
Any news when WMS, WMTS, WFS layers will be in ArcGIS API for JavaScript 4.x ?
https://developers.arcgis.com/javascript/latest/guide/capabilities/
Is there some workarounds at the moment?
Like publishing WMS service in ArcGIS Server service or Online Map?