this is an example that works, I tested it today:
const results = await Moralis.Cloud.run("watchEthAddress", {address: address, sync_historical: false}, {useMasterKey: true})
but you can not run it in front end, you could run it in cloud code, as it requires master key.
if a user authenticates with metamask, then you donât have to add that particular eth address to watch because it will be synced automatically