You can save the information in a foreach loop (e.g. in a List<>), and then open the form, using this information.
var myList = new List<something>();
foreach (DataRow iRow in chatcheck.Rows)
{
FormCollection fc = Application.OpenForms;
foreach (Form f in fc)
{
if (f.Text != ChatReader["Sender"].ToString())
{
myList.Add(...)
}
else if (f.Text == ChatReader["Sender"].ToString())
{
f.BringToFront();
}
}
}
foreach (var val in myList)
{
ChatBox chat = new ChatBox();
...
}