Connect the Air Pi tx pin to your flight controller UART RX pin.
This is the same UART you have selected in the configurator.
This UART might also be shared and/or labeled as the designated serial RX pin (serial RX, SBUS, ppm) it depends on your board. Keep in mind some boards have soft serial capability thereby allowing an uninverted serial connection to be made.
NOTE: softserial connections are limited to 19200 baud. This limits you to the msp protocol in Open.HD, which is msp v1. Users have had success with msp RC but beware.