Method POST Web API returns 'NULL'?

Question

I’m developing a Web Api with ASP with VB.NET to receive parameters JSON and created a method post with the following code.

Public Function PostValue(<FromBody()> ByVal value As String) As String
    return value
End Function

And this code has always returned to me null, Does anyone have any idea what it might be?

Observing: I’m using the APP Postman to carry out the requisition POST and this method will receive a string JSON.