Hello,
As Nicolas said :
it would require entering all the possible cities in the custom field and have it display as a dropdown, and then modify the shipping plugin to be able to restrict it on the selected city.
The solution will be to directly :
- Create an "Address" custom field via "Hikashop->Display->Custom fields"
- Edit the code of your shipping plugin to adapt the shipping price and availability regarding your customer shipping address and the custom field value
But some development skills will be required for the second one.