initiateTransaction method
Initiates a payment transaction with the provided paymentUrl on the native platform.
This method must be implemented by platform-specific subclasses to initiate the transaction
on the native platform, passing the provided paymentUrl, referrer and returning a result (e.g., toast message).
Implementation
Future<String?> initiateTransaction(String paymentUrl, String referrer) {
throw UnimplementedError(
'initiateTransaction(_) has not been implemented.');
}