1. Login to your Admin Area ( WHMCS Admin Area) 
2. Navigate to Addons-> XUI Reseller Dashboard 
3. Click on the " + Add Your New XUI Reseller Panel " button on the right top side.
4. Enter Your API Details ( API Key, Access Code, and Reseller Panel URL with Mag Portal ) 
5. Hit the Save Changes, it will test the API connection and save it.  

where to find the API Key of XUI in the Reseller Panel?

To get the API Key of XUI One Reseller Panel. Follow the below-given steps:

Login to the XUI One Reseller Panel.

Go to the Credits> User Profile> API Key.

and you will see the API Key for XUI one reseller panel in the last filed.