Paypal Integration: Add tax for 'user defined amount'option

Is there a way to add tax or paypal fees when using 'user defined amount' for payment?

It is not possible to add tax when using 'user defined amount' option. The tax option is only available for selling products. 

I will raise your request to add tax when using 'user defined amount' option to our developers, but we cannot provide you the exact time when and if this will be implemented. If there is any update we will inform you via this thread. 

For now, considering that you are using 'user defined amount' option, I can only suggest you to use basic fields outside of the payment field to let your users enter 'Payment Amount' and calculate the tax. Then you can pass total amount to your payment field.

How to Perform Form Calculation Using a Widget

How to Pass a Calculation to a Payment Field 

Here is my demo form: https://form.jotform.com/72836581735971