The iddata object is a structure, so in this instance, use cell array indexing and structure referencing to get its contents —
Fs = 500;
L = 50;
t = linspace(0, L-1, L).'/Fs;
u = [0 ones(1,L-1)].';
sys = tf([1 0], [1 0 1], 1/Fs);
y = lsim(sys, u, t);
% figure
% plot(t, y)
% hold on
% plot(t, u)
% hold off
% grid
k = 1;
data{k} = iddata(y, u, 1/Fs)
Input = data{k}.InputData % Return Input Data From Cell Array Of 'iddata' Objects
I created data to test this, since none were provided.
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