Your UPI ID of FASTag is NETC.VehicleRegistrationNumber@kotak .
For e.g. Vehicle Registration number is MH04BB1234 and issuer bank is Kotak Bank then UPI ID will be netc.MH04BB1234@Kotak.
1) You should get the UPI ID validated with your bank before recharging FASTag through UPI. Recharge can be done through UPI/ Internet Banking/ Credit Card/ Debit Card by logging into the FASTag Customer Portal.
2) Just login to the portal https://fastag.kotak.com/ using the credentials and load fund from any of the source mentioned above to your FASTag account. Or do a UPI to netc.RegisteredVehicleNumber@kotak from any UPI app like Google Pay/ PhonePe/ Amazon/ Kotak Bank Mobile App etc.