use the default inlet of an external

Hello everybody,

Since a few days, I’m learning how to build external in C
but i can’t figure out how can i use the inlet 0 ( default)
I’ve tried to add

void simplemax_in0(t_simplemax *x,long n); //prototype

and

void simplemax_in0(t_simplemax *x,long n){
post(“hello world! );
}

but the inlet 0 don’t understand int ( max window message)

How can i use it?

thanks