Posting array to WebApi action method

You could use a Dictionary like so:

public async Task<IHttpActionResult> Post([FromBody] Dictionary<string, int> employeeIds)

Then you could send a request like the following

Authorization: Bearer [token]
Host: localhost:44301
Content-Length: 16
Content-Type: application/json

{
    'first': 10,
    'second': 20,
    'third': 30
}

****EDIT****

In response to your update and comment on my answer, in that there is one key per array of numbers. You could use the .Net type Tuple like this:

public async Task<IHttpActionResult> Post([FromBody] Tuple<string, int[]> employeeIds)

you then access the values by code using

string myKey = employeeIds.Item1;
int[] theIds = employeeIds.Item2;

The request looks like this:

Authorization: Bearer [token]
Host: localhost:44301
Content-Length: 16
Content-Type: application/json

{
   Item1: 'OfficeWorkers',
   Item2: [10, 20, 30, 40, 50]
}

However this is obviously less declarative and the tuple is less obvious to others who use the api/modify your code in the future.

If it was up to me, I'd favour a small class (only existing in the API layer) which acts as a simple model which holds data received from the API calls - then in the WebApi code - map that to a "proper" class you are using internally.