How can I use customnet to define a nonlinearity estimator with the unknow parameters

When I use Hammerstein-Wiener model, I want to define a nonlinearity estimator with the unknow parameters,which as the picture shows.

 But I cannot run it rightly. I am confuse how to define the nonlinearity estimator with the unknow parameters. If it is unable to define this function, how the function of idpoly works which needs to  decide the coefficients.
The code uses customnet is as follows:
H1=@sigmoid_first;
InNet=customnet(H1,'NumberOfUnits',10);

The code of sigmoid_first is as follows: function [f g a]=sigmoid_first(x,coeff_beta)

f=coeff_beta(3)+coeff_beta(4)./(1+exp(-(coeff_beta(1)*x+coeff_beta2))); g=coeff_beta(1)*coeff_beta(4)*exp(-(coeff_beta(1)*x+coeff_beta(2)))./(1+exp(-(coeff_beta(1)*x+coeff_beta(2)))).^2; a=0.2;

Could somebody solve this problem? Thank you very much!