@model ProviderViewModel
<form asp-action="Create">
    <div>
        @Html.LabelFor(m => m.Name)
        @Html.TextBoxFor(m => m.Name)
    </div>
    <div>
        @Html.LabelFor(m => m.Url)
        @Html.TextBoxFor(m => m.Url)
    </div>
    <div>
        @Html.Label("Attributes")
    </div>
    @if (Model.Attributes.Count > 0)
    {
        @for (var i = 0; i < Model.Attributes.Count; i++)
        {
            @await Html.PartialAsync("_ProviderAttribute", Model.Attributes[i])
        }
    }
    <div>
        <input type="submit" value="+" formaction="AddAttribute" formmethod="post" />
    </div>
    <br/>
</form>

[HttpPost]
        public IActionResult AddAttribute(ProviderViewModel provider)
        {
            provider.Attributes.Add(new AttributeViewModel
            {
                Guid = Guid.NewGuid().ToString()
            });
            return View(nameof(Provider), provider);
        }