Hi,
As i understand from the question you are using contracts as a source of supply for your SC and the contract is entered at item level. when you click this button a new window will open up where you have a selection screen and once user clicks search, you are making a call to back-end ECC and fetching some data. Now this data needs to be populated in a specific item. Am i right?
Please attach some screen shots and the code u have written.