Sitefinity Thunder can detect custom properties but you should make sure that:
1. You have added a public property. Thunder cannot see field variables or non-public properties.
2. You have built your project after the last changes which you have made to the widget. Thunder looks for the widgets and their properties from the project's assembly.
If you cannot find your widget in the listed widgets while making the designer, please make sure that the widget does not have the ControlDesigner attribute and then build your project. Thunder filters out the widgets which have designers.
Please let me know if you continue to reproduce this issue.
the Telerik team
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