Create a variable and set a default value. Then override this if the field contains a value. Use this variable in the GetSQLValueString argument.
$myvariable = 8888888
if (!empty($_POST['fieldname'])) {
$myvariable =$_POST['fieldname'];
Create a variable and set a default value. Then override this if the field contains a value. Use this variable in the GetSQLValueString argument.
$myvariable = 8888888
if (!empty($_POST['fieldname'])) {
$myvariable =$_POST['fieldname'];