The easiest way is to use the sin/cos macro to create sine and cosine waves of the desired period and fit using the regression function. Suppose that the period of the cycle to be fitted is 5.92 and the series to be fitted is in F37.

5.92 Usc F37 R2

This will leave behind the regression equation on the stack which we can view with the T function. This will show three constants being the coefficients a, b, c in the equation F37(estimate) = a + b * sin + c * cos. The R function actually also displays the constant and coefficients.