OK, never mind. I figured it out. I really want to create an application first and then add a new target to it. The target is a Mail Extension. Hope this helps anyone else just starting out like me.

I am speaking from very little understanding, but from what I do understand is that the MailKit in its current form is extremely limited. I don't believe that there is any way to achieve what you want. There doesn't even appear to be any methods for manipulating existing messages. For example, I'd like to be able to move messages around to folders, add meta data to messages, etc. MailKit doesn't seem to enable this level of access mail.

Mail app already has the ability to change signatures of an email draft with a dropdown UI. You can set up as many signatures as you want.