Captive portalredirect for Authentication

We are using a UBR LTE that uses the captive portal to capture data and then needs to grant the user wifi access. Currently we our own external URL that has a form which the users need to complete and submit. The issue is how do we then get the form to send back the instruction to the UBR to grant wifi access.

I have read this article Configuring an External Splash Page for Captive Portal - #2 by rocknolds however I am not an developer and if somebody could possible assist me with the correct HTML for the submit button to redirect back to the peplink and grant access. Below is our form and the button needs HTML as per above article for this to happen. Please could somebody assist me.