How to add withholding tax to Paid Prepayment? Withholding Tax was missed to add during the creation

Summary:

Withholding Tax was missed during the prepayment creation. Now, the prepayment is already paid and accounted. How to update the prepayment withholding tax or how to update the withholding tax of the prepayment during prepayment application to standard invoice?

During prepayment application, Withholding Tax field of the Prepayment line is greyed out.

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