If you want make a Feild is required on a form in power apps portal follow the below steps:

1.Navigate—>Portal management app.

2.Open your Entity Form,which you want to set a “feild is required“option.

3.Next,Open Basic form metadata tab.Then Select your Basic form and Type: attribute. Next Choose a field to which you want to make required.

In general,select feild is required option.Next,Save the Record.

Now save and sync configuration and reload the Portal  page.

It Look’s like the below page.

Hope this Helpful!