Without having ran your code, I think the issue is you need to decorate your ApiControllers Action with the proper HTTP verb attribute.
public string GetRequest([FromBody]string value)
{
needs to look like
[HttpGet]
public string GetRequest([FromBody]string value)
{
And if you were trying to write an API action that utilized POST then you would add
[HttpPost] instead of
[HttpGet]
But one thing to note, for a Get request you'll need to remove the FromBody, so if this was meant to be a Post, then use
[HttpPost]