executing function from dll

prototype BOOL MyDll.MyFunction( BYREF STRING )

function dll return bool
passing argument byref string

how to call dll function ....
StrCpy $0 "abc123"
System::Call MyDll::MyFunction (t r0) b .r1

get data for $1 wrong help me please!
...thank you

Is your DLL mabe in .NET?

Maybe I'm wrong...is been a while, but as far I know, nsis can't call .net entries