Portal Site Collection:
A Portal Site Collection is used to link one site collection to another site collection. Portal site will be added to the Navigate Up (global breadcrumb navigation).
PowerShell Script:
$site = Get-SPSite "https://serverName:1234/sites/Sample/"
$site.PortalName = "Sample Link";
$site.PortalUrl = "https://serverName:1236/sites/Test/"
$site.Dispose()