POST Method Web API returns 'NULL'?

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

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

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

Note: I'm using the PostMan APP to perform the POST request and this method will receive a string JSON .