Add Lookup Field to SharePoint List using PowerShell

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue 

#configuration parameters
$WebURL="https://portal.crescent/Projects/"
$ParentListName="Parent Projects"
$ChildListName="Project Milestones"

$ParentLookupColumnName="Project Name"
$ChildLookupColumnName="Parent Project Name"

#Get the Parent and Child Lists
$Web = Get-SPWeb $WebURL
$ParentList = $web.Lists[$ParentListName]
$ChildList = $web.Lists[$ChildListName]

#Check if Field exists already
if(!$childList.Fields.ContainsField($ChildLookupColumnName))
{
    #sharepoint powershell - create lookup field
    $ChildLookupColumn = $ChildList.Fields.AddLookup($ChildLookupColumnName,$ParentList.id,$False)
    $ChildLookupColumn = $ChildList.Fields[$ChildLookupColumnName]

    #Setup lookup Field property
    $ChildLookupColumn.LookupField = $ParentList.Fields[$ParentLookupColumnName]
    #$ChildLookupColumn.AllowMultipleValues=$true
    $ChildLookupColumn.update()
    write-host "Lookup field added successfully!" -f green
}
else
{
    write-host "Field Exists already!" -f red
}