Using MPC Live as a midi controller and recording . How??

Hi Everyone. I searched for an answer but couldn’t find one that I understood. I know how to make a midi track to control (sequence) an external synth (or in my case iPad) and I can play an external Synth and record audio onto an audio track but how do I record the audio from
An external synth while using the MPC as the controller? I’d like to use the pad perform function on my mic live to control an iPad synth and record that into the unit. Any and all help appreciated for this newbie! Thank you!!