set another control value in other form

frmSearch fsearch = new frmSearch("formName", this);
fsearch.Show();

public frmSearch(string formName, Form  _form)
{
  switch  (formName)
     {
     case "client":
                   _form.txtBoxclient.Text  = value; //Error can not find control
                break;
                case "provider":
                   _form.txtBoxprovidet.Text  = value; //Error can not find control
}