> My question is, Need I set the value of the resistor in Port  to be 50 ohm?
> If not, what value I must set?

If DUT is perfect linear, we can characterize NF with arbitrary source impedance
since we can get constant {NFmin, Sopt, Rn}. So in this case, you don't need to set it as 50ohm.

But for mixer, this is not true since local power will be leaked to RF port
then cause some amplitude in source impedance.
This amplitude in source impedance could perturb operation point of mixer.
Even though there is no local leakage, source impedance value could
change operation point of mixer due to nonlinear nature.

So for mixer, you should set source impedance closely as actual value such as
expected output impedance of image rejection filter or output impedance
of driver amplifier.
If you assume actual measurement using NF-meter without image rejection filter,
source impedance should be 50ohm.