How to claim credit of form 1042 & 1099 for 15% royalty withhold tax by irs (us government)?

One of my client is a PVT LTD web design and development firm. They sell website design themes on this platform https://themeforest.net/ (Envato). Envato deduct 15% of our theme price if that is purchased by USA person. They will provide us Form 1042 / 1099 (which Envato submit to IRS - https://www.irs.gov) at the end of financial year and they suggested to contact our accountant to get tax credit of that amount of royalty withholding tax when we submit our income tax return to Indian government. Can someone please throw some lights about how we can claim tax credit of 15% amount which we paid already to IRS? Thanks in advance, CA M A Ravjani