undefined reference to WinMain@16 (codeblocks)

When there’s no project, Code::Blocks only compiles and links the current file. That file, from your picture, is secrypt.cpp, which does not have a main function. In order to compile and link both source files, you’ll need to do it manually or add them to the same project.

Contrary to what others are saying, using a Windows subsystem with main will still work, but there will be no console window.

Your other attempt, compiling and linking just trial.cpp, never links secrypt.cpp. This would normally result in an undefined reference to jRegister(), but you’ve declared the function inside main instead of calling it. Change main to:

int main()
{
    jRegister();

    return 0;
}