Because this is not official part of the Nuptial rite, the best place to have this dedication to Mary would be after the Post-Communion prayer, before the final blessing.
What couples did at the Cathedral (and I think the paractice is still used today) is to go to the statue of the Blessed Mother and place the bridal bouquet there and pray silently for a few minutes. Then, they retire to their place and the Mass concludes.
Unfortunately, there are some couples, mostly on the part of the bride, who, most regrettably, make a huge production number out of it for the sake of the video and cameras. There a long, pregnant pauses and dramatic looks and it doesn’t at all look sincere. It looks better suited for a wedding depicted on a Spanish TV telenovela.
Just be careful. Mean what you are going to do and don’t do it for the sake of the photo op.
