You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: invoices_client.go
    path of file: ./repos/azure-sdk-for-go/sdk/resourcemanager/billing/armbilling
    
    the code of the file until where you have to start completion: //go:build go1.18
// +build go1.18

// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License. See License.txt in the project root for license information.
// Code generated by Microsoft (R) AutoRest Code Generator. DO NOT EDIT.
// Changes may cause incorrect behavior and will be lost if the code is regenerated.

package armbilling

import (
	"context"
	"errors"
	"github.com/Azure/azure-sdk-for-go/sdk/azcore"
	"github.com/Azure/azure-sdk-for-go/sdk/azcore/arm"
	"github.com/Azure/azure-sdk-for-go/sdk/azcore/policy"
	"github.com/Azure/azure-sdk-for-go/sdk/azcore/runtime"
	"net/http"
	"net/url"
	"strings"
)

// InvoicesClient contains the methods for the Invoices group.
// Don't use this type directly, use NewInvoicesClient() instead.
type InvoicesClient struct {
	internal       *arm.Client
	subscriptionID string
}

// NewInvoicesClient creates a new instance of InvoicesClient with the specified v
    
func (client *InvoicesClient) downloadInvoiceCreateRequest(ctx context.Context, billingAccountName string, invoiceName string, downloadToken string, options *InvoicesClientBeginDownloadInvoiceOptions) (*policy.Request, error) {
	urlPath := "/providers/Microsoft.Billing/billingAccounts/{billingAccountName}/invoices/{invoiceName}/download"
	if billingAccountName == "" {
		return nil, errors.New("parameter billingAccountName cannot be empty")
	}
	urlPath = strings.ReplaceAll(urlPath, "{billingAccountName}", url.PathEscape(billingAccountName))
	if invoiceName == "" {
		return nil, errors.New("parameter invoiceName cannot be empty")
	}
	urlPath = strings.ReplaceAll(urlPath, "{invoiceName}", url.PathEscape(invoiceName))
	req, err := runtime.NewRequest(ctx, http.MethodPost, runtime.JoinPaths(client.internal.Endpoint(), urlPath))
	if err != nil {
		return nil, err
	}
	reqQP := req.Raw().URL.Query()
	reqQP.Set("api-version", "2020-05-01")
	reqQP.Set("downloadToken", downloadToken)
	req.Raw().URL.RawQuery = reqQP.Encode()
	req.Raw().Header["Accept"] = []string{"application/json"}
	return req, nil
}