Sorry if sounds noobish, but, when I was doing a "Friend wants to find out financial details of enemy" mission, I got the balance and clicked reply, but I was unable to add the balance to the reply email text. I tried putting it in the subject too.

I think if you hover over the email and type it in, that is how you do it.
(That is how it works in normal Uplink anyway, don't know if Onlink has implemented the "new mail system" yet)

I thought on another post Miah stated that a tab/selection method was implemented. I forgot where I found it, but I do remember seeing it.

Oh, thanks.
Turns out I needed to reinstall.
I had a corrupt version.