If you type in "Perth" it seems to recognise it for me - but don't worry if it doesn't auto-complete as you can just click on search when you have typed it in.

This would price up as two sectors added together. I am seeing one way BKK-PER as 22000 Avios + £ 47.70 in Y, and 55250 Avios + £ 54.00 in J. There are some additional pricing options in each case where you can pay a bit more cash to reduce the avios required.