Before you can add order IDs to your conversion tracking tag, you’ll need the following:

• Conversion tracking set up for your website. To follow the instructions below, you’ll need to have created a conversion action in your Google Ads account and added the conversion tag to your website.
• Unique order IDs, such as order confirmation numbers for your transactions. The order IDs can include numbers, letters, and special characters like dashes or spaces, with a character limit of 64 characters. They must be unique for each order. The order IDs must not include any information that could be used to identify individual customers.
• Ability to edit code on your website. Either you or your webmaster will need to edit the conversion tracking tag.

Instructions

You’ll need to add a piece of code to your conversion tracking tag to pull the unique order number and send along in your tag. You don’t need to update anything in your Google Ads account: just update your tag.

1. Open the ASP file for your conversion page. (Or, if you use a different server language, open the appropriate file.) If multiple pages are generated from the same file, find the section in the ASP file that generates your conversion page.
2. If your site has a variable order id, find the ASP expression which stores that information. For example: <%= orderId %>.
3. Open the HTML for your conversion page, and find your AdWords conversion tracking tag. (You can also make these changes to your conversion tracking tag before adding it to your conversion page. Edit the tag according to the instructions below, and then install the tag according to the instructions in Add a conversion tracking tag to your website.) 
4. In the script section of the tag (between the tags), add the following code, replacing <%= orderId %> with the ASP expression you found in step 2:
   
   if (<%= orderId %>) {
var google_conversion_order_id = "<%= orderId %>";
}
    
5. In the noscript section of the tag (between the  tags), add the following code to the src URL, replacing <%= orderId %> with the ASP expression you found in step 2:
   
   &oid=<%= orderId %>
6. To test the code, visit your conversion page by completing a conversion on your site.
7. In your web browser, view the source of the webpage. (In most browsers, you can do this by right-clicking the page and selecting View source). You should see the conversion tracking code between the and  tags on your page. The ASP expression will be replaced by the actual order ID.

 Below is an example of a conversion tag with the order ID code highlighted:


    ...
    
    
    
    
    

About Order ID and the "Count" setting

Adding an order ID to your tag is not the same as changing your “Count” setting to count one conversion per ad click.

By adding an order ID, you avoid counting the same conversion twice, such as when someone re-opens a purchase confirmation page and one purchase would be counted as 2 conversions.

When you change your “Count” setting to one, you count only one conversion after an ad click, even if multiple conversions, such as multiple different purchases, resulted from the ad click.

So when should you use “Count” versus Order ID? 

• If you want to track one lead per ad click, change your count setting to “One.”
• If you want to track every purchase a customer makes after an ad click, but want to make sure you don’t count the same purchase twice, add an order ID to your tag. Adding an order ID is the recommended option for purchases.

* Nguồn: Google