MikroTik does not natively support OAuth. You need a third-party RADIUS like WiFiDog or MikroTik Social Login script using API. Common approach:
Depending on your MikroTik configuration, the template should support: Hotspot Login Page Template Mikrotik
Modern MikroTik hotspot templates use CHAP (Challenge-Handshake Authentication Protocol) for security. The flow: MikroTik does not natively support OAuth
Some templates fall back to PAP (plain text) if JavaScript is disabled, but CHAP is preferred. Some templates fall back to PAP (plain text)