Try something like below. This code will work properly if there is no more than one treeView located on each open form.
My logic is not difficult to guess. I'm iterating all controls belonging to child form and if I find TreeView control I'm updating "Market" node.
foreach (Form f in Application.OpenForms)
{
if (f.Name == "form1")
{
foreach (Control ct in f.Controls)
{
TreeView treeViewToUpdate = ct as TreeView;
if (treeViewToUpdate !=null)
treeViewToUpdate.Nodes["Market"].Nodes.Add(Textbox1.text));
}
}
}