Delphi: This form of method call only allowed for class methods

I have two units in Delphi, each corresponding to a window in my program. In the first window, I defined a procedure, that I would like to call in the second window.

My first approach to call the procedure was the following:

MyProcedure(Sender);

However, with this approach, Delphi could not find the procedure, so I thought, I have to precede the name of the form to tell Delphi where the procedure is defined:

TForm1.MyProcedure(Sender);

However, this approach also does not do it. The program compiles, but when you open the program, the following error message occurs, with which I can not do anything:

This form of method call only allowed for class methods

I have already tried to define the procedure in the public section and such things, but it just is not working. Can someone help me?