Here are some formulas for displaying the ABN or ACN in Header Templates.
In your header template, you can create a line and use the following formulas depending on whether you want to display the ABN, ACN, both, or just one.
To display just the ABN:
{{#if Entity.ABN}}ABN {{Entity.ABN}}{{/if}}
To display just the ACN:
{{#if Entity.ACN}}ACN {{Entity.ACN}}{{/if}}
To display the ABN when only the ABN is specified, the ACN when only the ACN is specified, or just the ABN when both are specified:
{{#if Entity.ABN}}ABN {{Entity.ABN}}{{#elif Entity.ACN}}ACN {{Entity.ACN}}{{/if}}
**Note: else or elif statements also need a hash: ({{#elif}} or {{#elif}}
Refer here for the syntax/additional documentation: jehugaleahsa/mustache-sharp: An extension of the mustache text template engine for .NET. (github.com)