Air Carrier Captain reported undeclared Hazmat transported due to a communications breakdown between flight crew and Dispatch. The required NOTAC was not provided to the flight crew prior gate departure.

Narrative

I got distracted with the airplane being a MAX which caused me to forget to [ensure] I received the form Battery Powered Wheelchair or Mobility Device Form. I had discussed the form initially with the First Officer at the gate because I saw the customer in the wheel chair. Also the Flight Attendant asked me to make sure that the wheelchair was boarded and I made the comment 'I will know when it's boarded because I'll get the form.' However because I was getting reacquainted with the MAX I forgot to ask the agent for it. Also I would like to note that the Release may have the incorrect code for the wheelchair. The better code for the wheelchair should have been [a more appropriate code]. At least one of the wheelchairs had a battery because I saw the wheelchair at the gate and the customer sitting in it. I will make a notation on a paper to remind me in the future. It will also be helpful to remind the gate agents / ramp personnel we need the form. Unless we see the person in the wheelchair or the wheelchair itself we may not be aware we need the form.