Your items collection needs to filter the view, not just use the same view. This is the reason you are getting all of the branchs. What you are doing is creating what is called cascading parameters. You select a value for the first parameter (po number) and then the next parameter is filtered by the first (branch in your case).
So just to level set, your PO Number dropdown is set to:
Items : '[sbi].[PurchaseOrders_HeaderInfo]'
In order to get teh cascading behavior, the branch dropdown Items need to be based on a filtered set matching the selected PO Number, or;
Then in the branch dropdown property sheet "Value", select the branch number column as the value to display.
Now, when you select a PO Number from the first dropdown, the filter will be applied to the Branch dropdown and the list of only the branches associated with that PO Number should be included. And the branch dropdown should default to the first branch in the list of matching branches.
If you want something different defaulted, you can set the Default property with an expression that gets the value you want.
If you want the branches to be sorted by branch number for example, then add SortbyColumns to the Items expression;