Hi there, @charuksha ...
The Canvas Inbox currently has a pretty basic text editor (no functionality like what you'd experience using the Canvas RCE - Rich Content Editor). There is a long-standing request, however, for the Inbox interface to utilize the RCE. You can find that feature request here: [Inbox] Add rich-content editor to Conversations - Instructure Community - 357121. As you can see from this request, there is a lot of conversation from the past several years about this, so if you have some time, check out what folks are saying...as well as some of the feedback received directly from Instructure.
I know this isn't a direct answer to your question, but I hope that it helps in some way to shed some light on this kind of request.
