Hi @Lennard End ,
This option is not available in SharePoint 2019, but can run PowerShell code as a workaround.
Since we do not have a test environment for the subscription version at the moment, it is unclear whether the subscription version supports this option. You may try the 2019 solution.
You need to connect your on-premises SharePoint environment to Azure AD Access Control to configure federation between Azure Active Directory and on-premises SharePoint. The goal is to allow users to log in to Azure Active Directory and use their identity to access SharePoint on-premises sites. It requires Office Online Server as the guest sharing links leverage the web-based Office viewer/editor.
Here is the PowerShell code:
$farm = Get-SPFarm
$farm.Properties.Add("GuestSharingEnabled",$true)
$farm.Properties.Add("SPO-GuestSharingUIEnabled",$true)
$farm.Update()
Reference: Implement federated authentication between Azure Active Directory and SharePoint on-premises
Third-party reference articles: Enabling Anonymous Share Links in SharePoint Server 2019
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