To set up the cost of VAT function, select General -> Invoice settings -> Purchase invoice settings from the menu. At the end of the list you will find the fields:

The 22% code must be selected for the "50% VAT expense" main line VAT code and the 0% code must be selected for "50% VAT expense" turnover line VAT code. The choice can be made by clicking on the field:

The prerequisite for setting it up is that at least one VAT code with a 0% rate and one with a 22% rate has been entered into the VAT code register. After saving the action, the VAT cost and 50% VAT cost functions become active on the purchase invoice.

(The functionality is available only to users of the Premium package)