The "easy" way is to tape a spare key under the dashboard.
You used to be able to buy a module that would mount near/around the ignition that would "emulate" the keyfob's signature, but I haven't seen one of those for a while.
For what it's worth, I don't really recommend remote starts. They usually require some pretty invasive wiring, and (as you see here,) the bypassing of basically every anti-theft feature the car has.