FFT Harmonic Order

Hello everybody, I'm studying harmonic analysis in a voltage signal I have. I used the FFT from the gui environmental block in the simulink. The THD (Total harmonic distortion factor) that is calculated automatically from the window is calculated up until the 498th harmonic.

Does any one happen to know how to limit it to the 7th or the 15th harmonic, I want to calculate the THD up till the 15th harmonic not the 498th harmonic order. I can't seem to find the way to do that....

Thanks