When using a third party integration you need to make sure that it can use custom fields that has specific names like gender. I believe Ninja Forms can. In this case you will want to make sure that you have created the custom field in your Constant Contact account and then make sure Ninja Forms allows you to map that same custom field in their interface.
Since this is a third party integration I would suggest to reach out to Ninja Forms directly to get instructions on how to map the fields correctly after you have created it in your Constant Contact account.
Regards, Jimmy D. Tier II API Support Engineer
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