If you can select multiple lines in your window and want to be able to copy all of them to your clipboard, you need to loop through the lines and add a $crlf between them. Like this:
Code:

  Copy :{
  
    ;  Copy the first line to the clipboard without a $crlf (in case there's only just the one line selected
    ;  and not using the -a switch because we want to overwrite (initially) what's on the clipboard.
    clipboard $sline(@TEST,1)
  
    ;  Now get ready to loop through the rest of the lines, if there are any.
    var %i
    while ($sline(@TEST,%i)) {

      ;  Since we've made it into the loop, we must have found another line to add. Add a $crlf 
      ; (carriage return/line feed) to get to the next line and then the text found on the next line.
      ;  Copy it to the clipboard, this time using the -a switch to add it to what's already there.
      clipboard -a $+($crlf,$ifmatch)
  
      ;  Increment %i to see if there is another line.
      inc %i
  
    }
  }

Your lines are now on your clipboard, ready to be pasted somewhere (with Alt-V or using $cb or however you want to access them).