After the recipe runs the first time, edit the file that is created with a title such as #Pinboard Inbox. When the recipe runs it will create a markdown list under that header.
If
Any new public bookmark
This Trigger fires every time you create a new public bookmark. NOTE: excludes bookmarks marked to read.
Then
Append to a text file
This Action will append to a text file as determined by the file name and folder path you specify. Once a file’s size reaches 2MB a new file will be created.
Explore more great ways to automate Dropbox and Pinboard