You shouldn't have to specify which DLL to take them from, they're automatically available (I think). You should just be able to define the functions in an Extern block, a lot like you would in a decls file in BlitzPlus.
...at the top of your code is the equivalent of (from memory here :P)
Extern "win32" Function SendMessage:Int(hWnd:Int,MSG:Int,wParam:Int,lParam:Int) = "SendMessageA@16" End Extern
...at the top of your code is the equivalent of (from memory here :P)
.lib "user32.dll" SendMessage%(hWnd%,MSG%,wParam%,lParam%):"SendMessageA@16"