Add Site: Web Browser Authentication pop up page is blank!

Solution

Web Browser Authentication is needed if you are using:

Office 365 with Multi-Factor Authentication (MFA)
Using Active Directory Federation Services
Have a custom login screen for authentication
Using Single Sign On (SSO)

If you see a blank screen which does not allow you to enter any credentials, you may need to do one or both of these tasks below:

Add your SharePoint site URL to Internet Explorer as a Trusted Site or as Local Intranet (zone)
Change Internet Explorer settings to suppress security prompts and Login using current username and password. (This is due to the Web Browser Authentication pop up not supporting Windows Security prompts)

TASK 1: Trusted Sites

Adding to Trusted Sites will tell Internet Explorer (and your machine) to allow the web browser login page to show up in our pop up window as it should be 'safe'.

Open Internet Explorer, Internet Options
Click to select "Trusted Sites"
Click "Sites" button
Make sure your SharePoint domain/root url is listed

TASK 2: Automatic Logon using current credentials

If doing above still does not resolve the issue, you can test the scenario 2 by opening Internet Explorer (must be IE) using the same computer the tool is installed on, and navigating to the site. If you see a Windows Security prompt, then this means you need to suppress this prompt.

Example of prompt when navigating to SharePoint site using Internet Explorer.

This option below is one way to suppress this prompt if you can use the current username and password:

Change this value in the Internet Options:

Open Internet Explorer
Click the Gear wheel on top right
Click Internet Options
Click Security→Trusted Sites→Custom Level
Scroll all the way down to "User Authentication", choose "Automatic Logon with current user name and password"
Retry using the SharePoint Essentials Toolkit