Sometimes it is needed to add custom field in config. For example if you want to add Minimum amount for surcharge in config, you need to follow below steps-
Step1: Create a module by which we can overwrite system.xml file,Bellow is module file
Onsis : Namespace
Overwriteconfig : Module name
Step2: Then create bellow file.
In the above system.xml file, i have added “min_amount_for_surcharge” which will show in bellow path.
admin->system->configuration->Payment Methods -> Cash On Delivery -> Minimum amount for surcharge
Also we can get value of Minimum amount for surcharge using bellow code