view.cshtml

@if (item.Actions.Any(x => x.Rel == "ChangeQuantity"))
{
    <form action="@item.Actions.Single(x => x.Rel == "ChangeQuantity").Href" method="post">
        <input type="text" name="quantity"/>
        <button type="submit" class="btn">
            <i class="fas fa-plus"></i>
        </button>
    </form>
}