Word has two features that do what you want. (This is a Word forum.) The first is Content Controls. The second is MacroButton Fields. Both work well in Word and can be saved in AutoText or other QuickParts.
I have no idea whether either will work in Outlook but you can certainly try. Here is a link to my free Add-In to assist with inserting MacroButton Field Prompts.
Otherwise, you may have to compose your message in Word and copy-paste to Outlook or send as an attachment.