I want to make regression in Matlab by my own formula. Is it possible to do it simple way?

When we want to make logistic regression we write in Matlab:

 [b,dev,stats] = glmfit(x,[y ones(size(y))],'binomial','logit');
 yfit = glmval(b,x,'logit');

And this algorithm uses logistic function to calculate output. I want to do regression for formula

   W(n+1) = W(n) + exp(-k*n)*a*D(n)

where k and a are unknown values and I want to find them according to data, same way as b in logistic regression. How can I do this? If I knew code for logistic regression algorithm I would be able to write my own function, but unfortunately don't know how it works exactly. Any help would be appreciated! Thanks in advance!