Hello,
Check out this guide:
https://documentation.extremenetworks.com/ExtremeCloud_IQ_Controller/v10.02/ExtremeCloud_IQ_Controller_v10.02.01_Deployment_Guide.pdf?_ga=2.31032636.2009494871.1658837244-1461443537.1656515490
Page 79 starts documentation on configuring External NAC with external captive portal.
Thanks
-Ryan