This is an old thread so I am assuming you got it sorted out. The answer posted by @ChristianAbata is the method I use to add a new list item and fill in multiple fields for that item, I do put the __metadata line at the top of the body entry though. I however am trying use one http request action to add multiple items at once instead of having to enter the action multiple times. In my case I am trying to add multiple links to the quick launch navigation with no luck. I tried to wrap the 2 entries in [] as mentioned above but I got an error.
[{
"__metadata": {"type":"SP.NavigationNode"},
"Title": "LINK TEST 2",
"Url": "https://linka/"
},
{
"__metadata": {"type":"SP.NavigationNode"},
"Title": "LINK TEST 3",
"Url": "https://linkb/"
}]
A node of type 'StartArray' was read from the JSON reader when trying to read the start of an entry. A 'StartObject' node was expected.
clientRequestId: cafa6478-eb00-4e24-b3f3-96902f32229d
serviceRequestId: 96a69fa0-e03b-3000-4b57-d447aac80409