I can think of two options, call them and buy the ticket over the phone (they might charge extra for this) or find your itinerary in ITA Matrix and then book through BookWithMatrix.com:
1. Go to ITA Matrix.
2. Click "Advanced controls".
3. In "Outbound routing codes" and "Return routing codes" type C:A3+
4. In "Outbound extension codes" and "Return extension codes" type F bc=a|bc=b|bc=c|bc=d|bc=e|bc=f|bc=g|bc=h|bc=i|bc=j| bc=k|bc=l|bc=m|bc=n|bc=o|bc=q|bc=r|bc=s|bc=v|bc=w| bc=x|bc=y|bc=z
5. Search, find and select your itinerary and copy and paste it into BookWithMatric.com and follow the links to buy the tickets.