The 17-year-old is one of three riders selected for the two-day event in Thailand, alongside Sophie Hatch and Clemency Hughes.

The trio were due to ride in the team events (1.05m) on Tuesday, before the individual class (1.05m-1.15m) on Wednesday.

Mannell won selection after consistent results across state and national competitions, including leading junior horse at this year's Adelaide Royal Show.

"She's very dedicated. She gets up at 5.30 in the morning to work for one of the local horse trainers (Trevor Sutherland) and gives a few lessons. It's not just handed to her on a platter," Mannell's coach Christine Lucas said.

"She's always worked very hard at the craft and goes to as many clinics as she can to learn more."

Each team is required to draw out of a hat what horses they will ride at the Princess Cup, which Lucas said adds another dimension to the challenge for up-and-coming riders.

"It's a whole new experience riding a horse you've never ridden before and use your skills to get around the course, hopefully without any rails down," she said.

"It's not easy because some riders can't get on a strange horse (and perform), you have to quickly assess what the horse requires and it's quite a challenge for the young riders."

Mannell will compete against competitors from nine other countries in Bangkok.

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