advmemo.Bookmarks[bookmarkindex] := linenumber;

advmemo.Bookmarks[bookmarkindex]: integer;

advmemo.BookmarkIndex[bookmarkindex]: integer

procedure TForm1.FormCreate(Sender: TObject); begin
   // load some file
   advmemo1.Lines.LoadFromFile('somefile.txt');
   // set first bookmark at line 100
   advmemo1.Bookmarks[0] := 100;
   // set second bookmark at line 200
   advmemo1.Bookmarks[1] := 200;
end;
 // code to move to the first bookmark, i.e. jump to line 100; begin
  advmemo1.GotoBookmark(0);
end;