Form 1042 is proper form and you will need to withhold taxes form the payment. See attached IRS publication which is on point to your issue as to what to do and file, etc.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.
You should engage an experienced CPA or accountant to assist you in filling out Form 1042-S since it can be somewhat confusing, especially the income codes.
This response shall neither be deemed to create an attorney-client relationship nor constitute legal advice but rather is intended to provide general information about a complex legal issue.