how to find the horizontal Asymptotes of fitted curve equation for a data set

I have a data for (y,vor). I have used cftool to get fitted function for the input data. I want to know how to get the the horizontal asymptote of the fitted function as it approches zero (vor=0).

I have tried to use limit function but it does not work. I saved the fitted function as fittedmodel, then i used limit function to get teh asymtote

limit(fittedmodel,x,0)
Check for missing argument or incorrect argument data type in call to function 'limit'.