I can’t find a simple solution to this, but I probably don’t see the easy way: I want to select text in a markdown document and then insert the highlighted text AND the document reference into my clipboard so I can add both to another document I’m writing, maybe in the form of:
From [Document name|(reference):
selected text
How can I do this the easiest way?
Edit > Copy with Source Link.
I knew there was a simple solution like that. I just didn’t remember this or where to find it in all those loaded menus. I’m the victim here, being overstressed by all my screens and so. 
Wow! That’s so cool. I never thought about this being possible, assuming that if I copy something and then later want to paste it, how should DT know where I picked it up from. But it does and it works and this is a charme!
Inside DEVONthink Edit > Paste with Source Link is recommended as this command is able to format the output depending on the current document. Edit > Copy with Source Link is basically intended to paste to other apps.