Customized Credit memo template

You can use the Other field in the Credit memo template, Michael R. 

Aside from customizing or adding a field, you can check the Other box under the Additional Customization window. You can either add the sales order number in the in the Header or Column section. 

The Item Description and Memo fields on the credit memo is also helpful in tracking sales order number in a credit memo. When you run a sales report with items, you can add the Item Description and Memo columns. I have screenshots below for your reference where to enter the S.O number on the said fields and how to add these columns to a report. 

As an example on how to add them on the columns, let's run the Sales by Item Detail report:

1. Go to Reports > Sales > Sales by Item Detail. 
2. Click the Customize Report button. 
3. Under the Columns section type Item Description and check it. 
4. Search and check Memo. 
5. Click OK. 

Either of the ways on how to add a field for a sales order number, you'll have to manually enter, I also sent a product feedback to our engineers about this suggestion. 

If you have other questions you can always reach out to me. Mention me so i'll be notified. Thanks!