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Wix Mobile Apps Request: Adding Custom Fields to Your Login Screen

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Currently, it's not possible to add custom fields to the member log-in screen of your native app. 

This would allow you to request specific information, such as age, country of origin, a social media profile, or other business-related information, to people who are accessing your app for the first time. You can however, customize the signup form that appears on your desktop site.
We are always working to update and improve our products and your feedback is greatly appreciated.