Move event from personal calendar to shared calendar

I have events in my personal calendar that become relevant to others that I have a shared calendar with. The other user is the shared calendar owner. I can create an event in the shared calendar but I want to be able to edit the event I have already created and select the shared calendar. Currently it only shows my personal calendars when I try to do this edit.

I use proton calendar on both web and IOS 15.8.4 (I have an older iPhone) this issue persists in both versions.

It has been resolved in IOS 18 but I cannot use that.