grpc how to call method in service if i dont have the message response in proto file?

Question

How can I call the SendInvoiceXML method in InvoiceService.Grpc from the service, if there is no reply message in the SendInvoiceXMLReply type response (contained in the proto file)?

file.proto

syntax = "proto3";

option csharp_namespace = "InvoiceService.Grpc.Invoice";

package Invoice;

service InvoiceSender {
    rpc SendInvoice (SendInvoiceRequest) returns (SendInvoiceReply);
    rpc SendInvoiceXML (SendInvoiceXMLRequest) returns (SendInvoiceXMLReply);
}

message SendInvoiceRequest{}
message SendInvoiceReply{}

message SendInvoiceXMLRequest{
    string xml = 1;
}
message SendInvoiceXMLReply{}

InvoiceService.Grpc.Invoice.Service.cs

public override Task<SendInvoiceXMLReply> SendInvoiceXML(SendInvoiceXMLRequest request, ServerCallContext context)
        {
            return Task.FromResult(new SendInvoiceXMLReply
            {
                  //call method SendInvoiceXML in service here
            });
        }

InvoiceService.Service.Invoice.Service.cs

public string SendInvoiceXML(string xml)
        {
             //it is already implemented
        }