Can I book a flight for 90 days of onward travel when applying for a 60-day tourist visa for Thailand?

When applying for a 60-day tourist visa to Thailand, you must provide proof of onward travel. This typically means booking a flight that departs within the validity period of the visa. In your case, although you intend to stay for a total of 90 days by extending your visa for an additional 30 days while in Thailand, you still need to show proof of onward travel that aligns with the initial 60-day period of your visa application.