how do I call external functions from my nasm assembler .obj? or how do I link it in general with my project?

i created a header with the same name as the asm-file and defined the functions with:

extern "C" void __stdcall myfunc();

the asm-source is:

public _myfunc@0
_myfunc@0:
   .....
   ret

but the compiler/linker doesn't find the function. what did I do wrong?
thanks in advance :)