Here is a simple example using procedure nlin .

data test;
   a=2;b=2;
   do x=1,3;
      y=(1/( (1+x/a)**b) );
          output;
   end;
   
run;

proc print;run;

proc nlin data=test;
parms a1=0.1 b1=0.1;
model y=(1/( (1+x/a1)**b1) );
run;