i can't find documentation to config an external captive platform like mikrotik have (https://wiki.mikrotik.com/wiki/HotSpot_external_login_page), as you can see on this wiki, they specify all de parameters to receive on a external portal and the connection parameters to the router.
I would like to do this:
The user connect to the hotspot and automaticaly the system send a form via POST to a specific URL with some parameters in our case, the MAC of the client, the MAC or something to recognize the router, the URL that try to connect.
After that our platform process the received info and show the captive portal with a custom register and other things, when the user finish the register process, we have to send some parameters to de teltonika router to grant the internet access, in the mikrotik case we have to send http://192.168.80.1/login?dst=https%3A%2F%2Fwww.facebook.com%2Fclearchannelchile%2F&username=T-84%3A3A%3A4B%3AD6%3A26%3A58 (the IP is the gateway of the router, then the URL to redirect)
Hello sorry for late reply,
to use external captive portal set it up as external Radius server in general Hotspot settings, and for parameter values go to Services > Hotspot > URL parameters in grey color there is default names for parameters but if you fill them in you can make custom names for them to fit yours system.