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I believe you are looking to have this option (Please see image below) in your payment tool.
However, this option is not available if you selected Donations.
If you want you can create a separate textbox field where you can apply Form Conditions to include tax exemption. Or, you can also use a Form Calculation widget for the computation. Then, once you have done that, you can pass the calculation to the payment field.
Let us know if you have any questions about the instructions given above. Also, if this is not what you are referring to, please feel free to give us more information about your issue and we will be glad to help you further. Thank you.