Can the Rest API change the connection string (username/password) of a published dataset?

I know I can do it in PBI Desktop.  But that's no help to me.  The API claims to be able to update connections...

https://msdn.microsoft.com/en-us/library/mt748181.aspx

but I can't get it to work at all.  Keep getting '400 Bad Request'.  I'm starting to think the API doesn't work too well.