ALM Defect: Value change based on another value selection

Hi,

I am trying to build a logic where the value is Assigned To changes based on the "System component" value user selects. "System Component" is user defined field BG_USER_01 and is a user list.

For example: While tryin to log defect, if component selected is "User interface" then it will be assigned to "Mr. Smith" by default.

Where can I do this?

thanks.