The auto top-up option will be available for almost all the operators/countries. 

If the operator/country you want to send your top-up through is available, you will see the auto top-up feature in the Summary like this:

If it's not available you will not see the "Set up auto top-up" feature as above. 

If you have any question, please contact us through our Live Chat or email address: customercare@ding.com