How can I customize the purchase requisition page in 26B (Redwood)

Summary:

I need to default the value in a descriptive flexfield field that represent the Department of the requester.
I want to be automatically default the department based on the employee department that is creating the requistion.
Why? Because I need the department to define the natrual account, and also I want to avoid manual input from requesters.
Based on the department, the same expense Item, can impact differents natural account segment in the accounting.
We have around 65 requesters, with a volume of 15k month requests, so manual error can happens.

Example:
DEPARTMENT_1 + ITEM_ABC = ACCOUNT1
DEPARTMENT_2 + ITEM_ABC = ACCOUNT2