Code:

  Copy : clipboard $mid($sline(@TEST,1),1,$calc($pos($sline(@TEST,1),.,1) + 60))

That popup literally means this, when clicked:
  • Copy to the clipboard
  • the first selected line in the @TEST window,
  • starting from the first character
  • continuing through 60 characters past the first period on the line.
Collective's answer is most likely exactly what you are looking for.
Code:

  Copy: clipboard $sline(@Test,1)

This version means, when clicked:
  • Copy to the clipboard
  • the first selected line in the @Test window.
Stopping here is exactly what you asked for. Where did you get that first example from? It's pretty specific for a simple copy to clipboard command. Since you have found the /window command and therefore, the /help /window command, scroll down to the very bottom where the @Window $identifiers are described and read up on $sline().