@using System.Collections.Generic

Type here: <input onkeypress=@OnKeyPressed />
<ul>
    @foreach (var key in keysPressed)
    {
        <li>@key</li>
    }
</ul>

@functions {
    List<string> keysPressed = new List<string>();

    void OnKeyPressed(UIKeyboardEventArgs eventArgs)
    {
        keysPressed.Add(eventArgs.Key);
    }
}