The Object Reference ID is a system-generated identifier for every item and asset in your account.
To find the Object Reference ID for an item:
Navigate to the details screen for the desired item (see Search for an Item for more details on this process).
The first tab in the Tool Ribbon is typically named after the item type ("Campaign" for example). Click this tab to see the Item Details screen. This screen shows the "Object Reference ID" value.
For some item types, like FTP Profiles for example, the Object Reference ID isn't displayed anywhere within the Messaging user interface. For these items, you'll need to use the Search endpoint to search for the item. The Search response message will contain the Object Reference ID.