Hi Peter. Thanks for your request.
As you know, we currently do not have a TinyMCE / full HTML editor field type at wp-admin > Events > Settings > Additional Fields.
You could add this as a feature request at our plugins’ UserVoice page. I’d recommend describing your use case / reason for requesting the feature to help it along.
Adding your feature request there allows others who are interested in that feature to easily voice their support. We frequently review suggestions to find out which ones are popular, then we implement as many of them as we can.
Once you post it, feel free to link to it from here in case anyone comes across this forum thread in the future.
Thank you.