The "Walled Garden" option in the SSID access control page might be useful to you. It does not really redirect users to your webpage when they first connect to the SSID, however, it allows you to manually specify an IP range or domain names users can access prior to sign-on. I don't think it would be possible to redirect to your webpage first before the Meraki splash page.
One other option you can use is the "Where should users go after the splash page?" option in the splash page configuration page, you can use this to redirect users to your web page after they finish with the Meraki splash page.