Add a field to SharePoint custom list using CSOM PowerShell

Add-FieldToList.ps1

function Add-FieldToList()
{
    param(
        [Parameter(Mandatory=$true)][string]$url,
        [Parameter(Mandatory=$true)][System.Net.NetworkCredential]$credentials,
        [Parameter(Mandatory=$true)][string]$listName,
        [Parameter(Mandatory=$true)][string]$fieldName,
        [Parameter(Mandatory=$true)][string]$fieldXml,
        [Parameter(Mandatory=$false)][bool]$isExistingField
    )
    begin{
        try
        {
            #get Client Object
            $context = New-Object Microsoft.SharePoint.Client.ClientContext($url)
            $context.Credentials = $credentials
            $web = $context.Web 
            $context.Load($web) 
            $context.ExecuteQuery()

            #Retrieve List
            $List = $Context.Web.Lists.GetByTitle($listName)
            $context.Load($List)
            $context.ExecuteQuery()

            if($isExistingField -eq $true)
            {
                #Find an existing site column
                $field =  $Context.Web.AvailableFields | Where {$_.Title -eq $fieldName}
                $context.Load($field)
                $context.ExecuteQuery()
            }
        }
        catch
        {
            Write-Host ("Error while getting context. Error -->> " + $_.Exception.Message) -ForegroundColor Red
        }
    }
    process{
        try
        {
            if($isExistingField -eq $true)
            {
                #Add existing field to the List
                $List.Fields.Add($field)
                $List.Update()
                $context.ExecuteQuery()
                Write-Host "Field Added successfully" -ForegroundColor Green
            }
            else
            {
                #Add new field to the list
                $List.Fields.AddFieldAsXml($fieldXml,$true,[Microsoft.SharePoint.Client.AddFieldOptions]::AddFieldToDefaultView)
                $List.Update()
                $context.ExecuteQuery()
                Write-Host "Field Added successfully" -ForegroundColor Green
            }
        }
        catch
        {
            Write-Host ("Error while adding field. Error -->> " + $_.Exception.Message) -ForegroundColor Red
        }
    }
    end{
     $context.Dispose()
    }
}