How to automatically put a player in a minecart in Minecraft?

No, they have to actually click it to get in.

I would like someone with command block knowledge.

No not really you have do that manually

Not how it works- command blocks can't force a player to interact with something like that. You can move them around, move their inventory around, give them buffs/debuffs etc…, but not force them to interact with something.

It's like saying 'can I make them fire a bow without them clicking on it', the game just can't make it happen.

Plus, there's the possibility that people playing on cheat-enabled servers could abuse this function if it were to exist, and the minecraft developers would be against it.

Sorry, if you want someone to get in a cart, you'll have to just set them near it and encourage them to jump in.

^-^ Hope this helped! ^-^