Enable Foreground:
$Configuration=(Get-WSUSServer).GetConfiguration()
$Configuration.BitsDownloadPriorityForeground=$true
$Configuration.Save()

Disable Foreground:
$Configuration=(Get-WSUSServer).GetConfiguration()
$Configuration.BitsDownloadPriorityForeground=$false
$Configuration.Save()

Check Foreground State:
(Get-WSUSServer).GetConfiguration().BitsDownloadPriorityForeground