powershell equivalent of stsadm -o setproperty -pn peoplepicker-searchadforests
$wa = get-spwebapplication <>
$searchad = $wa.peoplepickersettings.searchactivedirectorydomains
$newdomain = new-object Microsoft.SharePoint.Administration.sppeoplepickersearchactivedirectorydomain
$newdomain.domainname = <domain/forest DNS name>
blogs
$newdomain.Isforest = $true/$false
$newdomain.loginname = <loginname>
[System.Security.SecureString]$secureStringValue = Read-Host "Enter the service account password: " -AsSecureString;
$newdomain.setpassword($securestringvalue)
$searchad.add($newdomain)
$wa.update()