Great. For now I do not need to use REST API then. I have some Shelly devices where I might want to poll power consumption using their REST API so I do not have to wait until they push updates to OH. But this is something for later. It is good to know to use sendHttpXRequest Actions when I get to it.
There is a slight mistake in your example, “addGroup” should be “addGroupName”:
ir.getItem("MyItem").addGroupName("OpenItems");
After you pointed me the right way I found this old post of yours with a lot of good information. For some reason I did not come across this before. There is also the information on “addGroupName”:
Thanks a lot!