@grandpalarry777,
Thank you for reaching out. I was able to sign in by adding exceptions for MightyText in Brave’s content settings. To resolve the redirect issue, go to Settings --> Additional --> Privacy/Security --> Site Settings --> Cookies, then add the following to the Allow list:
https://[*.]mightytext.net/https://[*.]mighty-app.appspot.comhttps://[*.]textyserver.appspot.com
After this, I was able to login to the extension and proceed with the installation process.