asp.net core custom model binder just for one property

I have a simple model for my asp.net core controller:

[HttpPost]
public async Task<DefaultResponse> AddCourse([FromBody]CourseDto dto)
{
     var response = await _courseService.AddCourse(dto);
     return response;
}

My model is :

 public class CourseDto
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Genre { get; set; }
    public string Duration { get; set; }
    public string Level { get; set; }
    public string AgeRange { get; set; }
    public string Notes { get; set; }
    public bool Active { get; set; }
    public string OrganisationCode { get; set; }
}

I'm trying to set value of "OrganisationCode" using a custom mode binder or action filter, but had no success. I would be thnakful if you advise whats the right way to updat ethe model before executing the action.

Thanks.