If you are using Postman to test the Web API then that has nothing to do with VB.NET itself, and you can configure them right from the Postman interface. Request body is provided for every request type, and in the same panel or tab that you are using to perform all the actions.
See this link to learn how to add request body to a request,
https://learning.getpostman.com/docs/postman/sending_api_requests/requests/#request-body[
^], you can also specify the kinds of request body you want to send, JSON, XML, or binary content.
If you mean you want to accept the data in VB.NET based Web API, then please check how to read the data from the
HttpClient[
^] object (or the type of the server framework you are using).