It looks like SoapUI Open Source doesn't offer the possibility to use WS and WSS connections. Are there any plugins that I can use for this? Has anyone done this before?
You mean WebSocket?
If so, please check this out
Thank you for your answer!
I already found this repo but it only supports WS (that means it doesn't support secured websockets). Additionally, it looks like the repo is not updated for years now. It might be a thing to update it a little bit.