Since freqz plots are ordinary subplots, just address them as such —
Fp = 1000; %Bandpass Freq
Fs = 2000; %BandStop Freq
SF = 10000; %Sampling Freq
Pr = 0.1; %Passband ripple
Sr = 0.001; %Stopband ripple
[n,fo,ao,w] = firpmord([Fp Fs],[1 0],[Pr Sr],SF);
b = firpm(n,fo,ao,w);
figure
freqz(b,1)
Ax1 = subplot(2,1,1);
yline(Ax1, 0, '-r', 'LineWidth',2)
The horizontal line at y=1 is going to be a bit difficult to see, so I emphasized it. Change the yline call appropriately if necessary to change its appearance.
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