I'm stealing the info from this post from this excellent tutorial:
https://www.ronnipedersen.com/2017/07/11/enable-modern-authentication-for-skype-for-business-online
I'm simplifying for my future use. Important note - the DNS records as defined by Microsoft should be in place before going through this process. Particularly, there should be a CNAME record pointing lyncsicover.domainname.com to webdir.online.lync.com.
https://aka.ms/vs/15/release/VC_redist.x64.exe
This is needed for the Skype for Business Powershell installer.
Then download the Skype for Business Powershell installer itself:
https://download.microsoft.com/download/2/0/5/2050B39B-4DA5-48E0-B768-583533B42C3B/SkypeOnlinePowerShell.Exe
Then run PowerShell and run these commands:
$sfboSession = New-CsOnlineSession -UserName user@domain.com
Import-PSSession $sfboSession
Then run:
Get-CsOAuthConfiguration
You'll find a line for "ClientAdalAuthOverride" which will likely show "disabled" which is the behavior we want to change.
Run this command:
Set-CsOAuthConfiguration -ClientAdalAuthOverride Allowed
This process allows ADAL (aka modern authentication). The tutorial I am replicating says this change can take up to 24 hours to take effect, but it often takes effect within one hour.