Using Matlab functions in custom Simscape blocks

I want to make a custom Simscape block that uses the MATLAB function square. However, for some reason the Simscape language does not allow the use of MATLAB functions. When I try to run the model with the customized block, I get the following error:

 Error compiling Simscape network for model pg_stiffness.
 Caused by:
 ['pg_stiffness/Simscape Component']: No function or value named square.
 In spring_custom (line 31)

When using the MATLAB Function Block in Simulink, a similar error can be bypassed by declaring the MATLAB function as extrinsic . Is there a similar solution for MATLAB functions in Simscape blocks?