Probably their discords is best. You can find them by going to the Servers section of the Forums and going to Domains. All of the kingdoms posts should be stickied and will contain the details you need to get in touch.

discords is most direct, each kingdom usually has a discord.

What server are you playing on?

It's kind of tragic the way kingdom information is buried in hard to notice forums on the website when this is at the heart of the very game that's being made. The studio really should give much greater prominence to the information because players want to - and need to - know about who exists in the world. It's exhausting how often this question gets asked.

In case you're not sure where this stuff is at, on the main forums page, scroll down to the "SERVERS" section near the bottom.

Click into the server forum that pertains to you. From there, you should be able to find info on the various kingdoms and their respective duchies. Most use discord as the primary form of communication.

You can try posting in their recruitment threads or try private messaging them here in the forums, but Discord is the best and most immediate means of getting into contact with anyone.