Can I ensure 50% males and 50% females within my participants?
The best way to ensure this would be to run 2 studies of the same size that point to the same study URL. On one of these studies you should add a male only prescreening, on the other a female only. However, please note that the completion URL will be different for these studies so please provide both at the end of your survey.
You can do the above for any demographic where you want an even split.