Inside your Request Management Business object, when you highlighted Configuration Item Requested, do you get a "Manage Actions" option in the middle panel? If you do, select it and try creating an "Update" action called Update Requested CI. Now, in your process use that action at some point or multiple points in your process where you want the Requested CI updated. Now, with this in place, as long as you have an item in your Requested CI field, it should automatically set values based on what you added in the action.
This may be what you already tried, and if you did, my apologies for the same steps.
Some screenshots might be useful as well if possible.
Added attachment and screen shots. I may not have explained the use case very well.