Ok. So let's start from a client-side code.I suppose that you have a next Index.cshtml view.
<!-- You use this code to display data from your model -->
<input name="Y11" value="@Model.first" /> --------??
Than you need a code that post new Point object from your view to controller.It could be like something like this:
<form asp-controller="Home" asp-action="InsertPoint" method="post">
X value: <input type="text" name="x"><br>
Y value: <input type="text" name="y"><br>
<input type="submit" value="Submit">
In your controller you should create action with following signature
public async Task<IActionResult> InsertPoint(Point point)
//Validation and insertion to list
It's not an ideal solution. You could perform this task in many different ways. My aim, is just to show you the basic idea how it could be done. If you need more information you could start from this article
And of course, keep in mind that google is your good friend.