Change Profit Center before PO creation

[wwc0220]

Dear Experts,

I want to change the profit center when I click the save button to create purchased order.

I try to use Badi ME_PROCESS_PO_CUST - PROCESS_ACCOUNT method. But it is not work.

I checked the coding part, when the program execute the set_data method, there is a default value 'X' for the my_cust_firewall_on  parameter. It cause the update failed. 

No idea how to change this field correctly.