Hello Abbott, 

The reason why this issue occurs is that for .NET Core we do not provide our controls via the DLLs, instead they are provided by NuGet packages.

 In order for the Infragistics Windows Forms controls to show in the toolbox and to be able to use and interact with them in the designer when using a .NET 8 project, you need to first include the NuGet package for the particular control(s) you are trying to use, as our support for .NET Core is done through NuGet. For more information on setting up the Infragistics NuGet feed and which controls exist under which packages, I would recommend taking a look at the following documentation: 

NuGet Feeds: https://ko.infragistics.com/help/winforms/nuget-feeds   

NuGet Packages: https://ko.infragistics.com/help/winforms/nuget-packages   

Once the controls are added through the NuGet they will appear in the toolbox and would be able to interact in the form designer. 

Please let me know if you have any questions. 

Regards,
Ivan Kitanov