Activating subscriptions using Auto Actions

You can access trigger variables (you'll need to provide what trigger you're using for me to help here), get variables, etc.. using substitutions. For example [get_name] would give the result of &name=NAME_HERE in the URL. I highly recommend enabling format functions and using the "clean" format function to ensure the data is safe. You can find more details on format functions under the Parameters tab of your auto action.

I am triggering my own event with two parameters, user id and plan id. I believe these will be var1 and var2 in Auto Actions. Correct? My question is how I reference them in the SQL script. Is it [var1] and [var2]? I assume I also have to enable substitutions for the action.

Yes, if that's what's in your event variables then [var1] would be user id and [var2] would be plan id. You can just substitute those right into your query as long as substitutions are enabled under the Parameters tab (they are by default).