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Add a reference to the Wordbee.Beebox.Extensibility assembly. You find this dll in the Beebox install directory. If your Beebox does not run on your development PC, simply copy the dll from the server. By default this dll is found in:
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you can download it from here.
This is how it should look like:
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See the following pages for details, start reading here.
Compile and add to Beebox
The last step is to compile your project and upload the dll from the Beebox Administration pages, by navigating to "Administration" > "Extensions". Read more.