The Latitude fare codes for international flights are Y and B. You need to enter the following for the routing codes for the YUL-MXP and AMS-YUL segments:

AC+ /f bc=b|bc=y

AC+ specifies one or more flights on specific carrier (Air Canada)

/f bc=b|bc=y forces the search to only look for booking classes B and Y. Substitute other letter codes for other booking classes and add more |bc=* for additional letter codes (where the * is replaced by the letter code).

O:AC would force the operating carrier to be Air Canada on a direct flight (no code shares or subsidiary carriers). This could be in place of AC+, but would be more limiting.

Not adding any codes for the MXP-AMS portion should show the lowest fare for this segment. However, some low cost airlines will not be displayed (as they are never shown in the ITA Matrix results).