Detailed documentation for writing plug-in DLLs has been added to the XWorkplace User Guide with V0.9.9. Please see that documentation for details.

Still, since you have obviously downloaded the XWorkplace source code, if you are interested, I strongly suggest that you create the XWorkplace code documentation. Then, in the HTML files which are generated, look for the documentation for src\shared\center.c documentation which gives you plenty of introductory information.

Several standard XCenter widgets have been created as plugin DLLs to show you how this can be done. Their sources are in the src\widgets directory. In that directory, you will also find ____sample.c, which you can use as a template for your own widgets.