Sandbox Entities

• Launch S&box Editor
• New Project (top right)
• Select Sandbox Mode Addon

  

• If the Sandbox scene isn't open, go to asset browser, click on Sandbox on the left, then search for sandbox.scene and open it
• Pressing play should let you play Sandbox like normal
• In Asset Browser, in your addon's Asset folder, right click and go to new > Sandbox > Sandbox Entity
• Name it whatever you want

  

• Do it again, but create a new prefab.
• Select your Sandbox Entity and drag the prefab into the Prefab field and press SAVE
• Your SENT will now be spawnable in the game when you press play, under the Local tab

You can do whatever you want in your prefab now. If you want people to drag it around make sure it's got a Rigidbody.

When you publish, if you added custom code, like custom components - then tick the Include Code checkbox.

Object reference not set to an instance of an object.
System.NullReferenceException: Object reference not set to an instance of an object.
   at Editor.AssetList.DrawAssetEntry(VirtualWidget item, AssetEntry asset) in C:\program files (x86)\steam\steamapps\common\sbox\addons\tools\code\Editor\AssetList\AssetList.cs:line 361
   at Editor.AssetList.PaintListItem(VirtualWidget item) in C:\program files (x86)\steam\steamapps\common\sbox\addons\tools\code\Editor\AssetList\AssetList.cs:line 298
   at Editor.BaseItemWidget.PaintItem(VirtualWidget item)
   at Editor.BaseItemWidget.OnPaint()
   at Editor.ListView.OnPaint()
   at Editor.Widget.InternalPaintEvent(QPainter e, Int32 flags)
   at Managed.SourceTools.Exports.Editor_BaseScrollWidget_InternalPaintEvent(UInt32 self, IntPtr e, Int32 flags)

install facepunch.sandbox

Braxen make sure that in your .csproj file you have
<ItemGroup>
    <Using Include="Sandbox.Internal.GlobalGameNamespace" Static="true" />
    <Using Include="Microsoft.AspNetCore.Components" />
    <Using Include="Microsoft.AspNetCore.Components.Rendering" />
</ItemGroup>
and maybe
<ItemGroup>
    <ProjectReference Include="D:\Games\Steam\steamapps\common\sbox\addons\base\Code\Base Library.csproj" />
</ItemGroup>
Because for some reason they disappeared from my addon project and I returned them.