Passing Multiple Parameter Values via a Razor Page

Update your HTML form to something like the following:

<form method="post">
    <input type="hidden" asp-for="Course.CourseID" />
    <input type="hidden" asp-for="Course.InstructorID" />
    <input type="submit" value="Delete" class="btn btn-danger" /> |
    <a asp-page="./Index">Back to List</a>
</form>

Create a new class for handling the composite key. You can use your existing view model class too.

public class DeleteCourseRequest {
  public int CourseId { get; set; }
  public int InstructorId { get; set; }
}

Then update your controller's OnPostAsync action to something like this:

public async Task<IActionResult> OnPostAsync(DeleteCourseRequest request) {
  if (request == null) {
    return NotFound();
  }

  Course = await _context.Courses.FirstOrDefaultAsync(c => c.CourseID == request.CourseID && c.InstructorID == request.InstructorID);

  if (Course != null) {
    _context.Courses.Remove(Course);
    await _context.SaveChangesAsync();
  }

  return RedirectToPage("./Index");
}

The issue is that when doing a Post with more than one parameter, you would either have to use [FromBody] or [FromUri] bindings on your parameters. This tells ASP.NET Web API where to parse the parameters. Another option is to use a view model to pass your information from your form to your controller.