@model ModelB

<form method="post">
    <div class="col-sm-4">

        <label asp-for="Name" class="control-label"></label>

        <input asp-for="Name" class="form-control input-lg" />

        <span asp-validation-for="Name" class="text-danger"></span>

    </div>

    <div class="form-group row">
        <div class="col">

            <label asp-for="@Model.modelAs[0].FirstName" class="control-label"></label>

            <input asp-for="@Model.modelAs[0].FirstName" class="form-control" />

            <span asp-validation-for="@Model.modelAs[0].FirstName" class="text-danger"></span>

        </div>



        <div class="col">

            <label asp-for="@Model.modelAs[0].LastName" class="control-label"></label>

            <input asp-for="@Model.modelAs[0].LastName" class="form-control" />

            <span asp-validation-for="@Model.modelAs[0].LastName" class="text-danger"></span>

        </div>
    </div>

    <div class="form-group row" id="AAA">
        <div class="col">

            <label asp-for="@Model.modelAs[1].FirstName" class="control-label"></label>

            <input asp-for="@Model.modelAs[1].FirstName" class="form-control" disabled />

            <span asp-validation-for="@Model.modelAs[1].FirstName" class="text-danger"></span>

        </div>



        <div class="col">

            <label asp-for="@Model.modelAs[1].LastName" class="control-label"></label>

            <input asp-for="@Model.modelAs[1].LastName" class="form-control" disabled />

            <span asp-validation-for="@Model.modelAs[1].LastName" class="text-danger"></span>

        </div>
    </div>


    <button type="button" onclick="Additem()">Add new item</button>


    <button type="submit">Submit</button>
</form>

@section Scripts {
    @{
        await Html.RenderPartialAsync("_ValidationScriptsPartial");
    }

    <script>
        window.onload = function () 
        {
            document.getElementById("AAA").style.display = "none";
        };

        function Additem(){
            document.getElementById("AAA").style.display = "";
            var collection = document.getElementsByTagName("input")
            for (var i = 0; i < collection.length; i++) {
                collection[i].removeAttribute('disabled');
            }
        };
}