Full example

using BepInEx;
using MegaUI.Api;
using UnityEngine;

namespace ExampleMegaUiMod;

[BepInPlugin(Guid, "Example Mega UI Mod", "1.0.0")]
[BepInDependency(MegaUiApi.ModGuid, BepInDependency.DependencyFlags.HardDependency)]
public sealed class Plugin : BaseUnityPlugin
{
    public const string Guid = "ExampleMegaUiMod";

    // The state belongs to the mod, never to the UI. The widgets read from here
    // and write back here - which is why the screen can never fall out of step
    // with other code that changes these values.
    private static bool _warn = true;
    private static int  _limit = 5;
    private static int  _clicks;

    private void Awake()
    {
        var app = MegaUiApi.Register(Guid, new AppDefinition
        {
            Title = "Example",
            Icon  = UiIcon.Builtin("gauge"),
        });

        if (!app.Ok)
        {
            Logger.LogError($"Mega UI refused this app: {app.Problem}");
            return;
        }

        app.Build(ui =>
        {
            ui.Header("Example app");
            ui.Label("Everything on this screen was written with the Mega UI API.");

            ui.Gap();

            ui.Card(c =>
            {
                c.Row(r =>
                {
                    r.Icon("timer");
                    r.Label("Frames since the game started");
                    r.Spacer();
                    r.Value(() => Time.frameCount.ToString("N0"));
                });

                c.Row(r =>
                {
                    r.Label("Times the button was clicked");
                    r.Spacer();
                    r.Value(() => _clicks.ToString());
                });
            });

            ui.Header("Settings");

            ui.Toggle("Warn me about things", () => _warn, v =>
            {
                _warn = v;
                Logger.LogInfo($"warn = {v}");
            });

            ui.Stepper("How many at most", 1, 20, () => _limit, v => _limit = v);

            ui.Separator();

            ui.Row(r =>
            {
                r.Label("Ready?");
                r.Spacer();

                r.Button("Do the thing", () =>
                {
                    _clicks++;
                    Logger.LogInfo($"clicked {_clicks} time(s), limit {_limit}, warn {_warn}");
                });
            });
        });

        Logger.LogInfo("Loaded - open the store computer and look for 'Example'.");
    }
}

<ItemGroup>
  <Reference Include="MegaUI">
    <HintPath>$(GameDir)\BepInEx\plugins\rodopoulos\MegaUI.dll</HintPath>
    <Private>false</Private>
  </Reference>

  <Reference Include="BepInEx">
    <HintPath>$(GameDir)\BepInEx\core\BepInEx.dll</HintPath>
    <Private>false</Private>
  </Reference>

  <!-- Only because the example reads Time.frameCount. A mod showing its own
       data does not need even this. -->
  <Reference Include="UnityEngine">
    <HintPath>$(Managed)\UnityEngine.dll</HintPath>
    <Private>false</Private>
  </Reference>
  <Reference Include="UnityEngine.CoreModule">
    <HintPath>$(Managed)\UnityEngine.CoreModule.dll</HintPath>
    <Private>false</Private>
  </Reference>
</ItemGroup>