If you click the reply button on a post (or start a new thread) rather than use the quick reply box below, you get a range of buttons along the top.
Catch the URL you want to direct to by (for example) highlighting and copying it from the address bar while viewing it.
You must get the whole thing including the http part.

In the message compiling box you can paste in this URL.
Some forum software will automatically recognise it as a link, but here expect to need to make it work like this.
Highlight the URL you pasted in here, them click the link button - which only becomes active when you have something highlighted.
A small window should appear - paste the URL again into the appropiate place and click insert.
It should then be a clickable link - try preview to check.
You can also make regular words into a link like this - Go to AgTalk home page