You can link customers and suppliers to your accounting package in two ways. The first is through the customer/supplier page, (we recommend using this method for Sage One in particular):
The 'Third party accounting ref' refers to a code that can be found usually in the URL of a customer or supplier's profile within the corresponding accounting software.
The other is within your company settings. Go to 'Accounting integration' and from here, click into 'Quick links'. You will then have the option to link customers/suppliers:
(Please note that this method does not work for Sage One).
When you open this settings page, you will be able to paste the accounting reference from your accounting package into the 'Accounting reference' column. Ensure you hit 'Save accounting references' afterward.