in the UI withholding is calculated by a client script and field access is disabled. you cannot override any value

Thank you so much. In that case, I believe we cannot push this tax value from external system. Is there a way how to handle withholding tax to be pushed from external system?

oh no. in the UI you are stuck..if you bring in via CSV or externally you can set the value. Read up on CSV imports of Withholding tax in the Help Menu