Captive portals on routers use http redirect. If an unauthenticated client requests a website, DNS is queried by the browser and the appropriate IP resolved as usual. The browser then sends an HTTP request to that IP address. This request, however, is intercepted by the router and forwarded to a redirect server. This redirect server responds with a regular HTTP response which contains HTTP status code 302 to redirect the client to the Captive Portal. To the client, this process is totally transparent. The client assumes that the website actually responded to the initial request and sent the redirect.
In case of https, interception is not possible, because it would break the security of the layer.
So in most cases it is not possible to redirect https request, however this can vary by the hardware and firmware you are using, some may allow this.