Report: Setting value for filter field


In the reports: I need to set the value of filter field from javascript or python, I need it to be set automatically based on the selection of another filter field.

Example: I need to set the customer_name field once the customer field is selected.

Normally we do this in the document using cur_frm.add_fetch('customer', 'customer_name', 'customer_name') but how can we do this in the reports code?