I havent had to charge my wifi controller yet while out flying my Mavic 2 Pro. I have the DJI mobile battery charger for the Mavic 2 to charge the batteries. It does not include a cable to charge the controller. What do you all use or recommend to charge the wifi controller in the field?

Any micro usb charger or cable should work.  

Use the power bank adapter with USB

Hi, Thanks for the inquiry. I would like to clarify if you're trying to charge your RC of the Mavic 2 Pro while flying the drone? Please keep us posted for further clarification. Thank you for the support.

Power bank charger cables might  work but please let us know!
powerbanktalk.com/best-power-banks This blog has some AC adapter power bank with specifications it might help you