Ooops. My apologies.
You can exceed 13 characters, but not quite as I typed. Sorry my brain increasingly has disconnects and I make mistakes I don't notice. What I should have typed, which is different is
CQ SOTA/VK3AFW QF22
I' m sure I posted this and some other suggestions before along with the acknowledgement that the average bear might also get confused if they have not heard of SOTA. And there are a lot of very average bears on FT8.
If you want to see what you can send, type the text in message 5. You have to shut the rig down and let the program transmit so you can see what is viable - the TX text will show in the RX window if the right boxes are ticked..
The FT8 program has shorthand methods of sending some elements such as 73, hence you can send VK3ZLP VK3AFW RR73, which is 18 characters, as a routine thing. Having a fixed format is also helpful for minimal coding but does impose some restrictions.
I've yet to figure out how to send my call and the SOTA Ref in the same message.
Unfortunately no response to my request for replacing the Grid Square in some transmissions with XOTA where X can be S, J, I, L, all common activities where FT8 would be useful.
Maybe it's too hard or considered plain silly.