Using a Autotask contact UDF value to specify which Autotask contact is sync'd to a Xero customer.
In the hierarchical relationship between Autotask and Xero. My Accounting Toolbox treats Autotask as the parent and Xero as the child.
It's recommended that Autotask contact data is always kept up-to-date when making updates to Xero contact data.
To assist our sync tool in selecting the correct Autotask contact we recommend setting up the UDF field below. If you need support setting this up please contact our support team.
Navigate to User-Defined Fields in Autotask
Autotask - > User Admin -> Features & Settings -> Application-Wide Features -> User-Defined Fields

Now select the Contacts Tab

Then click the New button

You have two options on UDF setup.
Option A: Free Textbox
Option B: Dropdown list (Recommended)
Please select one of these options to setup for the UDF
Please only select one of the two optional setup methods!
Option A: Free Textbox Setup
Now using the example below fill in the details - leave default value and variable name blank
- Name = Position
- Active = Ticked
- Field Type = Text (Single Line)
If you would like to use a dropdown list, please see option B below.

Or Option B: Dropdown list (Recommended)
This example is an alternative option if you want to use a dropdown list instead. So long as the Name = Position and the Value = Accounts.
Description and display value can be whatever makes sense in your business environment. You can populate this list with other variables if you wish it will not interfere with the sync.
Now your UDF is setup, you can now apply the "Accounts" position to your contact.
From the Autotask CRM, find your Account - in the example below we are using ABLE.
Click the link to bring up the Account window.
Click on the more toggle > then select Contacts

Find the contact that you want to send invoices to in Xero. Please only apply this to 1 contact per account.

Right click and select Edit Contact

Select the User Defined tab and select the Xero Invoice Contact under Position. Click Save and you are done!
Option A
Option B

Your new UDF not showing in your Autotask Account?
From the Autotask PSA menu, navigate to Admin→ Features & Settings→ Accounts & Contacts → Account Categories
1. Select the Account Category and click edit (eg standard)
2. Select the details tab
3. Scroll down until you see the Hidden Fields section
4. Find your UDF then click Make Field Visible
Save & Close, now retry your account the UDF should be visible.