Phone number Normalization for Jumpseller Checkout

phone_number_normalization.liquid

<!--Copy this code at the end of the Checkout Layout-->
<script>
  $(document).ready(function(){
    // Restrict to only numbers
    $('#order_customer_phone').keypress(function(tecla){
      if(tecla.charCode < 48 || tecla.charCode > 57) { return false;}
    });
    //Phone format
    $("#order_customer_phone").attr("maxlength", "8").attr("minlength", "8"); // Change the number according to what you need
    $("#order_customer_phone").attr("placeholder", ""); // you can add a format example
    $("#order_customer_phone").attr("title", "You must enter at least 8 numbers");
  })
</script>

cómo podemos hacer para que se "prellene" el campo con un 56 o +56?