How can I customize Email template edit form?

Hi, all.

I am using MS Dynamics CRM 2016 on Premises.
I want to add ribbon button to email template entity form, but I cannot do it by using RibbonWorkbench(it doesn't see the entity) or by manually editing the XML scheme (<ribbonnotsupported/>).
Is it even possible at any way?
Or can I add button to the rich email template editor panel?

Any ideas will be helpful.
Thank you.
With regards, Yuriy.