Using elements of a Form within a function

Question

I am developing an application in C++ Builder that has a Form which contains elements such as ListBox, buttons, Labels, etc..

I can configure the events of each of these elements (onClick, onEnter, onExit...) inside a file .cpp, but when I try to create a function within this same file .cpp access the properties of the elements (Visible, Enabled, Caption, etc.) get an error:

"Undefined Symbol".

How do I use the elements contained in a Form within a function created by me.

Follow the problem code.

//Função criada por mim, dentro da Unit1.cpp que contem o Form1 no qual está o ListBox1

void funcao(AnsiString parametro){ 

 ...
 ListBox1->Items->Add("ItemParaListBox"); //Essa linha gera o erro "undefined symbol ListBox1"

}

//Função de fastcall realizada para edição de evento do elemento dentro da mesma Unit1.cpp

void __fastcall TForm1::BitBtn11Click(TObject *Sender){

...
ListBox1->Items->Add("ItemParaListBox"); //Funciona normalmente

}