How to measure S11 of antenna using VNA for one port S-parameter measurement?
Connect the antenna port to the VNA port, measure S11.

You will want to ensure that there are no RF reflectors within sight of the antenna. This could be done by pointing it at the sky, out of a window, or at an RF absorber. Any signal reflected back to the antenna will appear as a returned signal at the antenna port, and so alter your true S11 measurement.