Which plumbing adapter to join 3/4" FIP line to 3/4" Female NPT inlet port?
You'll want a 3/4" nylon hex nipple. Commonly they're available in white, gray, and black, though other colors may be available. More importantly, you'll want to make sure the fittings are NSF/ANSI 61 compliant. NSF/ANSI 61 certifies that the fitting is suitable as a drinking water system component.