@Derelict:
Two choices:
Let connections to https sites hang and make your users have to connect to an http site to get the portal page.
Intercept https connections, display your portal page, and deal with all the ramifications of being an https man-in-the-middle with certificate errors, browser protections, installing your certificate as trusted for your user's bank because they clicked the wrong thing, HSTS, pinned certificates, etc.
Third choice:
Disable captive portal. Let the users connect and go.
Thanks I too option 1 I added https://google.fr and it works.