EViews.com

Hi i´m working on a non-linear relationship between the GDP growth rate(gy) and the initial GDP(year zero(yo)) for a group of countries. The main objective is to estimate the rate of convergence in different periods of time. I´m having trouble in the specification, how should i specificate it on eviews?

Equation: gy = b0 -((1-exp(-b1*20))/20)yo

Thanks in advance :D