Sounds like a job for a template variable!

Screenshot attached of one called pkRichText, inserted in my form with
[*pkRichContent*]. The Widget part of the definition needs to be set to "Rich Text", though the dimensions don’t seem to have any impact on the front-end display. (My default content is a placeholder that is filled when the form in question is redisplayed after an error)
The default setting for TinyMCE will be a minimal formatting toolbar below the text box. This can be changed (e.g. to a moderately powerful toolbar with alignment, insert images etc.) via Manage Resources > Plugins > TinyMCE > Configuration, selecting what you want for the Web Theme parameter.

KP