The XBee always uses an 8-bit serial setting, with the option (controlled by the NB parameter) of adding a 9th parity bit to that. If you want 7 bits and even parity to be transmitted, then you would use the 8-bit no parity setting on the XBee, and the sender would need to calculate the parity in the 8th bit.
If you check the BD and NB commands in the product manual, you’ll find all the details.
However, by my logic, what you’re after is the default setting anyway - so no commands are needed, unless you’re restoring the setting after having changed it to something else.