Don't quote me on that...
1) Highlight the required text in the original post
2) Press the Ctrl and C keys. ( i.e. copy the text)
3) Open the Reply box
4) Press the Ctrl. and V keys ( print the copied text into the reply box)
5) Highlight the copied text again
6) Press the "speech balloon" as described already.
QED.
( now - posting photographs, I haven't cracked that yet despite reading all the advice ! )