The easiest option in version 4 is actually a script for the Script with Input/Output action. Afterwards the result can be easily used via the Script Output placeholder. The next release will include such a script.
on scriptOutput(theRecord, theInput)
tell application id "DNtp"
if record type of theRecord is markdown then
set theText to plain text of theRecord
set theMetadata to get metadata of markdown theText
return |zettel-id| of theMetadata
end if
end tell
end scriptOutput