Hi Aron
Thank you for contacting Syncfusion Support.
Query : Do you know how i can resolve the issue without adding the dll manually to my solution but only by NuGet.
We hope you have added only Syncfusion.Tools.Windows and Syncfusion.Shared.Base assembly to add our control in ToolBox, Since some of our control will also depended on Grid assemblies such as Syncfusion.Grid.Base and Syncfusion.Grid.Windows, you need to add this dll also to make all the control to be visible in ToolBox.
Screenshot shows the following assemblies should be selected along with Tools.Windows and Shared.Base assemblies.
If the above assemblies are added then the MultiSelectionComboBox will be displayed in the below dialog window.
After selecting the item, the control will be attached in ToolBox. Please refer below screenshot.
Please try this solution and let us know it is resolved at your end.
Regards
Senthil