Dorico user config files are under c:\Users\<username>\AppData\Roaming\Steinberg\Dorico 6 on Windows and /Users/<username>/Library/Application Support/Steinberg/Dorico 6 on Mac.
Inside that folder there might be a folder called Script Plug-ins – if not, you can create it. (Note the hyphen in “Plug-ins”.)
Unzip the zip file and place the folder it contains inside Script Plug-ins. If Dorico was open, then restart it.
Now when you go to the Script menu in Dorico, you’ll see the “Add Second Ending Tie” folder, with the scripts inside it. Select the second ending note on which you want the backwards tie, and then select one of the scripts, depending on whether you want the tie above or below the note, and how long you want it.
The scripts basically add an l.v. tie and then go into Engrave mode to flip and extend them so that they go backwards.