There is no API call is available which matches to your use case. Alternatively you use some of calls of Trading API for adding, deleting and retrieving the items from a user's watch list. Please find the details of calls below:
1. AddToWatchList: Use this call to add one or more items to a user's My eBay Watch list. The My eBay Watch list can be viewed on the user's My eBay page, or the list can be retrieved using GetMyeBayBuying.
3. GetMyeBayBuying: Use this call to retrieve information from the All Buying section of the authenticated user's My eBay account. You can review items that the user is watching, bidding on, has won, has not won, and those items on which they have made best offers or second chance offers.