How can I run a command that requires user input through oh-my-zsh alias?
Aliases are used as a shortcuts - sipmple commands that expand to longer ones.
If you need to pass an argument to it, you would need to create a function.
Maybe try something like:

reminder () { automator -i $1 add_reminder.workflow' }


Here $1 will expand to the first argument that you've passed to the function.
So you'll to do something like:

reminder "Buy potatoes"


P.S: I haven't tried it, since I don't have zsh with me right now, so if it breaks, give me a shout.