
1 - Create dimension values for the customer source dimension code.
2 - Select the Search icon, enter Chart of Accounts, and then select the related link,
3 - From the Dimension code, select the Account Type Default Dimension list.
4 - Select the Dimension Value code that is used for the customer source.
5 - Set the Value Posting to Code Mandatory
Reference:
https://stoneridgesoftware.com/how-to-set-up-required-dimensions-in-dynamics-365-business-edition/