There are 2 types of additional fields you can add to your contest.
Signup Fields
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Signup fields are added to the contest signup in addition to the name / email fields. They look like this.
They are set-up in Sign Up (viewing) tab.
Then choosing Sign Up and then Custom Fields
Bonus Actions
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Bonus Actions, on the other hand, are asked AFTER the signup. These reward extra entries to your contestant.
When you're editing your contest, click the Thank You tab.
Then click the button to "Add / Manage Bonus Actions"
You'll be able to set-up and edit your bonus actions on this page!