I've followed the tutorial, but I'm not sure how to extract the single value I need. Where is it in the Matlab structure? (see "accessing FOOOF model parameters" in the tutorial)

If you are only interested in the frequency that has the maximum power between 8 and 12 Hz, you don't need any model to your spectrum, and you probably don't need FOOOF.

PsdMat = in_bst_timefreq('sepi01/@rawtutorial_eeg/timefreq_psd_230117_1242.mat');
for iChannel = 1:size(PsdMat.TF,1)
% Find the indices of the frequency bins of the alpha band
iFreqs = find((PsdMat.Freqs >= 8) & (PsdMat.Freqs <= 12));
% Find the maximum power
[maxPower, maxIndex] = max(squeeze(PsdMat.TF(iChannel, :, iFreqs)));
% Get the frequency value that has the max power
maxFreq = PsdMat.Freqs(iFreqs(maxIndex));
% Display the result
disp(sprintf('Channel %s: maxFreq=%1.2fHz, maxPower=%g', PsdMat.RowNames{iChannel}, maxFreq, maxPower));
end