Is there anyway to pass parameter in fit() function ?

I am trying to fit signal using

fit(t,y,'fourier3')

The coefficient I'm getting are having higher value than expected (x10^6). I have a feeling that its due to wrong frequency (w) being used in fit. I know the frequency of the signal. is there any way to pass the known frequency into the fit function ? In cftool, I am able to do it by specifying the w lower and w higher as same value. Can that be done in fit function too? How do I go about it.