You could have something like (not tested):
StringBuilder sb = new StringBuilder();
sb.AppendFormat("{0}-{1}-{2}-{3}-{4}",
this.RBLstSeries.SelectedValue.ToString(),
this.RBLstDamper.SelectedValue.Substring(0, 3).ToString(),
this.RBLstStyle.SelectedValue.Substring(0, 1).ToString(),
this.RBLstFrontLipDesign.SelectedValue.Substring(0, 1).ToString(),
this.RBLstApronDesign.SelectedValue.Substring(0, 1).ToString());
this.TxtHdModelNo.Text = sb.ToString();
Refer to
StringBuilder Class[
^] and
StringBuilder.AppendFormat Method (String, Object[])[
^]