How do you get your baby to drink from a straw?

How do you get your baby to drink from a weighted straw cup? We normally use the 360 cup but she refuses to hold it herself. She will hold the weighted straw cup but can't figure out how to drink from it. She knows to put her mouth on the straw but not sure she knows to suck.

She's 9 months and we introduced both cups around 6.5 months.