My 17 month old doesn't like to be restrained (ie: in a car seat, highchair, shopping cart, stroller or anything restrictive). He will screams like a banshee, arches his back and thrashes his arms and legs while throwing anything within reach. We have tried giving him a toy, singing a song, distracting him with food or sippy cup...etc. Saying no and ignoring it doesn't work either. He will continue to scream until we pick him up. Often, the tantrum continues when he is unrestrained.
It is really common for this issue to come up right around that 18 month point. It is at that age that we see the immersion of will. Your child is now at an age where he knows what he wants and doesn’t want but he does not yet have the complex of vocabulary to be able to talk through it effectively. There are a few things you can do to help the situation.
Understand, there are times where you have to maintain a boundary, like a safety restraint, despite all the kicking and screaming and all you can do is give him the words to help him understand all the big feelings he is having. The good news is that they usually grow out of this stage relatively quickly.