Similar to projects, contest payments will be credited to your Freelancer.com account under their listed currency. The currency of your payment will follow the currency that is set up on the contest prize.
Here is an example:
If the contest you participated in was posted with a USD prize, you will be paid in USD for winning the contest (or for being a runner up). The option to select the currency in which the prize will be credited to your account is not available.
Currency exchange rates fluctuate, so crediting the contest prize under its posted currency ensures that the posted value of the contest is awarded properly to the winner or to the runner-up(s).