SharePoint Online: Add Column to List Content Type using PowerShell

#Load SharePoint CSOM Assemblies
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.dll"
Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\16\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"

Function Add-ColumnToListContentType()
{ 
    param
    (
        [Parameter(Mandatory=$true)] [string] $SiteURL,
        [Parameter(Mandatory=$true)] [string] $ListName,
        [Parameter(Mandatory=$true)] [string] $ContentTypeName,
        [Parameter(Mandatory=$true)] [string] $ColumnName,
        [Parameter(Mandatory=$false)] [bool] $IsSiteColumn=$True
    )

    Try {
        $Cred= Get-Credential
        $Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.Username, $Cred.Password)

        #Setup the context
        $Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
        $Ctx.Credentials = $Credentials
    
        #Get the List
        $List = $Ctx.Web.Lists.GetByTitle($ListName)
        $Ctx.Load($List)
        $Ctx.ExecuteQuery()

        #Get the content type from list
        $ContentTypeColl = $List.ContentTypes
        $Ctx.Load($ContentTypeColl)
        $Ctx.ExecuteQuery()
        
        #Check if the content type exists in the list        
        $ContentType = $ContentTypeColl | Where {$_.Name -eq $ContentTypeName}
        If($ContentType -eq $Null)
        { 
            Write-host "Content Type '$ContentTypeName' doesn't exists in '$ListName'" -f Yellow
            Return 
        }

        #Get the column to add: Either site column or existing list's column
        If($IsSiteColumn)
        {
            $ColumnColl = $Ctx.Web.Fields
        }
        else #List Column
        {
            $ColumnColl = $List.Fields
        }
        $Ctx.Load($ColumnColl)
        $Ctx.ExecuteQuery()
        $Column = $ColumnColl | Where {$_.Title -eq $ColumnName}
    
        #Check if given column exists
        if($Column -eq $Null)
        {
            Write-host "Column '$ColumnName' doesn't exists!" -f Yellow
            Return
        }
        else
        {
            #Check if column already added to the content type
            $FieldCollection = $ContentType.Fields
            $Ctx.Load($FieldCollection)
            $Ctx.ExecuteQuery()
            $Field = $FieldCollection | Where {$_.Title -eq $ColumnName}
            if($Field -ne $Null)
            {
                Write-host "Column '$ColumnName' Already Exists in the content type!" -f Yellow
                Return
            }
    
            #Add field to content type
            $FieldLink = New-Object Microsoft.SharePoint.Client.FieldLinkCreationInformation
            $FieldLink.Field = $Column
            [Void]$ContentType.FieldLinks.Add($FieldLink)
            $ContentType.Update($false)
            $Ctx.ExecuteQuery() 
       
            Write-host "Column '$ColumnName' Added to '$ContentTypeName' Successfully!" -ForegroundColor Green
        }
   }
    Catch {
        write-host -f Red "Error Adding Column to Content Type!" $_.Exception.Message
    } 
}

#Set parameter values
$SiteURL="https://crescent.sharepoint.com"
$ListName="Projects"
$ContentTypeName="Projects"
$ColumnName="Head Count"
$IsSiteColumn=$false

#Call the function
Add-ColumnToListContentType -SiteURL $SiteURL -ListName $ListName -ContentTypeName $ContentTypeName -ColumnName $ColumnName