If you don't know how to link to it on the current thread
I think you'll find the correct term is 'if you don't know how to activate the text'. Which does appear to be quite difficult on the app. In a browser you just copy the url of the new thread, go back to the old thread and paste it into a message.
symbols optional.
Some other myths we need to kill:
That other posters will be cross if you mistakenly create it in the wrong topic - we would just ask Mumsnet to move it
That you have to be a genius title writer - nope - the priority is having a thread and if you look back over PseudoBadger's illustrious history of thread-starting, not every thread title was a marvellous kaleidoscope of puns.
That if someone offers you a title you have have to do some final editing to make it even more marvellous - nope, as above, all anyone really wants is a thread to post on.
That you have to be hovering over your keyboard when the perfect number of posts is reached - nope, I think I declared that I was happy to start a thread but I might have to do it early to avoid the current thread filling up before I got back to mumsnet
Frankly, even if it was just called, The Archers, possibly mis-spelled, and posted in the Etiquette topic, we'd all just be delighted that someone had made a lovely new thread 
Notdead I had exactly the same thought about Alice yesterday!