From the help files . . .
you can create your own custom template in plain text, rich text, or Markdown formats. These files can be created externally and added to ~/Library/Application Support/DEVONthink 3/Annotations.noindex or created in DEVONthink and exported via File > Export > as Template to the Annotations.noindex folder.
I did this. I made it a markdown file and export it as template into the annotation.noindex folder to create. But the problem is it does not lower me to select the template file if I am working with a TXT file. It is grayed out.