Cast On in middle of row

Good morning,

I am stuck! I am knitting a young boy’s sweater and am now doing the shoulder edging of Sirdar Pattern 1811. I have started ribbing (K3,p3)at the neck edge and on the 5th row, I am asked to:

 Rib 9, (cast on 1 st) twice, rib 9[/I][/B].  

How do I cast on? Can I use an m1? I await your expert advice.

Thank you so much for the help (past and present).

Joce

Is it for a buttonhole, or just an increase? You can use the backward loop m1 which should work fine either way.