How do you get a clickable path to a file or folder?
You might want to consider using a scripting language to do this. I've created a simple JavaScript for Automation program that can do the job (it can also be fairly easily replicated in AppleScript):

finder = Application('Finder')
finder.includeStandardAdditions = true
file = finder.chooseFile()
path = String(file).replace(' ', '%20')


You could then use the path variable to create a new OutgoingMessage in Mail.

Alternatively, you could use this as a "Run Javascript" block in Automator, then use its result in a New Mail Message (in which case you would need to append return path to then end of the script).