Using the MATLAB Function block, we can create a complex multiplication block with the required number of inputs.
Create a functional block in simulink, by double clicking on this function block write the below function which takes 2 real inputs and 2 imaginary inputs.
function y = fcn(a,b,c,d)
y = c1*c2;
The following MATLAB function block is created with 2 real inputs and 2 imaginary inputs. The multiplication operation of the two complex numbers are performed and the output is shown with the variable y.
Hope this helps..!