New Teams Won't Load Website Tab

ezybao I've just had a similar problem trying to add a website tab that displays a list of group forms, using the URL I'd captured from the Forms page.
In Classic Teams it works perfectly - when the Forms page loads in the tab, the login/authentication process kicks in and can be completed, allowing the Forms to be displayed.
In New Teams, the locin/authentication process simply doesn't work. The only option is to click the link to open the Forms page in a web browser. Seems this is a deliberate feature in New Teams - Microsoft 'announced' it earlier this year (but it passed me by): https://techcommunity.microsoft.com/t5/microsoft-teams/teams-retires-client-ability-to-load-websites-from-channel-tabs/m-p/4036930