Credit for this belongs here, but this is the script I use to create a new DT3 markdown item by placing it in the “new Item” tab:
tell application id "DNtp"
set newItem to create record with {name:"$title", type:markdown, plain text:"[$title]($user_link)"}
set refURL to reference URL of newItem
set itemPath to path of newItem
end tell
open location refURL
get refURL