The .NET controls used in a COBOL Winform application are the same controls used in a C# or VB Winforms application as they all use the underlying .NET Framework class assemblies. The behavior in COBOL is no different than the behavior in C#.
I do not see the demo program that you reference. Can you please attach it?
You can explicitly set the cursor position within a textbox by using the SelectionStart and SelectionLength properties of the TextBox class.
The following example will set the cursor position to the end of the field when focus is given to the control.
set textBox1::Text to "ABCDE"
set textBox1::SelectionStart to textBox1::Text::Count
set textBox1::SelectionLength to 0
invoke textBox1::Focus
You should probably google the term "winform textbox cursor position" and see what it comes up with as there are a number of different caveats to this depending on how you enter the textbox controls on your form.
Thanks.