You can use S-Function Builder block. This allows you to integrate existing C/C++ code into Simulink. The S-Function Builder block creates a S-Function wrapper for your external code, enabling you to generate a FMU from it. Refer here for more info:
Do ensure that the source code of the S-Function block is on MATLAB path so that Simulink can access it. Refer to this MATLAB Answer for more info: https://www.mathworks.com/matlabcentral/answers/826290-how-to-export-simulink-model-which-contains-s-function-block-mex-file-to-stand-alone-fmu
If your S-Function is on MATLAB path but MATLAB doesn't seem to find it, refer to this for a possible resolution: https://www.mathworks.com/matlabcentral/answers/735042-s-function-does-not-exist-but-is-clearly-in-the-matlab-path
