How to get data frombody in a custom ModelBinder?

I'm creating a custom implementation of IModelBinder in a .NET Core 1.1 (full framework) app.

I have a class ActivityModel that has an interface property:

public class ActivityModel
{
    public int Id { get; set; }
    public string Title { get; set; }
    public int Type { get; set; }
    public IActivityContent Content { get; set; }    // marker interface  
    //more unimportant props
}

The idea is that the client can send this object with a Content property that can be one of many (currently 8, but it may be growing) different classes.

This way we can have one endpoint in the API (for each CRUD operation) that can handle all of these different types of objects.

I'm in the process of implementing a ModelBinder to turn the object sent from the client into a concrete class, and here's what I've got so far:

    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext == null)
        {
            throw new ArgumentNullException(nameof(bindingContext));
        }
        var content = bindingContext.ValueProvider.GetValue("content");

        return TaskCache.CompletedTask;
    }

I'm currently just trying to get the Content object into a variable so I can work with it more, but ValueProvider only has RouteValueProvider and QueryStringValueProvider. How do I get the body data in here?