Hi,
Please refer to our Origin C document about how to Include Header Files and Access an External DLL.
In your issue, you can try to put both your "MyDLL.h" and "MyDLL.dll" in the folder "OriginC\X-Functions", which is under the User Files Folder. In the "MyDLL.h" header file, need the following line to specify the location of the dll.
#pragma dll(MyDLL, header) //in the same folder as this .h file
Then in the X-Function, include the header file like:
#include "MyDLL.h"
Penn