Follow the below steps to apply tax on an open item (items not available in catalog).
1. Create a general item with sell price 0 and map into the right tax category. This item can be used to bill open items.

Administrator Menu > Maintenance > Resources
2. In the resource, event.addline, change the below line, line number 31 in the default configuration:
line.setSalePrice(newPrice);
to
line.setSalePrice(newPrice/(1+line.getTaxRate()));

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