To Set Attribute:
Set-QADUser "userID" -ObjectAttribute @{extensionAttribute13='email address'}
To View Attribute:
Get-qaduser "userID" -includeallproperties | select extensionattribute13
Set-QADUser "userID" -ObjectAttribute @{extensionAttribute13='email address'}
To View Attribute:
Get-qaduser "userID" -includeallproperties | select extensionattribute13