To be able to call and use an external DLL function using CitectVBA, you must have previously provided CitectVBA with details about the function being called. CitectVBA requires information like the name of the function, where that function is located, what arguments it expects, and what type of data it returns. CitectVBA uses the Declare statement to detail that information.
The Declare statement must be positioned in the CitectVBA file in your project above and before any code that calls that declared function of the DLL.