I have Linux Mint 21, then I install qtcreator through apt repository. I want to activate Qt Quick Designer Plugin, based on this link, I should be able to activate it, but I didn't found QmlDesigner.
About Plugins Screenshot
I solved this by trying qtcreator from qt official instead from apt. Then QmlDesigner is there.
Note: You need qt account to install this.
Thank you, hope this help you too.