>There seems to be a difference between when I manually click on the button or either try to use form's submit or call click() event of the button in code.
>
>In the first case scenario, the passed string has

<btnRemoveOverdue>Remove+Overdue</btnRemoveOverdue>

inside it and in the other case it doesn't.
>
>I am wondering if that string in the post string comes from some code in C++ application or it's a standard browser behavior? Is it possible to somehow add this part to the passed post string?

Yes, a post can be a combination of FORM action attribute plus the form fields. So, you can adjust the FORM action attribute to be something like MyForm.aspx?Session=1,FirstName=Test and those would be added to the post as well.