To install our plugin manually, just copy the plugin to /FileMaker Server/Web Publishing/publishing-engine/wip/Plugins/ folder. Then check in Admin Console to make sure you have plugins enabled for the Data API. The Admin Console doesn't list plugins for Data API. If the plugin is not yet available, you may need to restart the wip process or the whole server to recognize the new plugin.
The exact location is for macOS:
/Library/FileMaker Server/Web Publishing/publishing-engine/wip/Plugins/
And for Ubuntu Linux:
C:\Program Files\FileMaker\FileMaker Server\Web Publishing\publishing-engine\wip\Plugins
If you have no toggle for Data API plugins in your Admin Console, you may need to get a newer FileMaker Server version.
/opt/FileMaker/FileMaker Server/Web Publishing/publishing-engine/wip/Plugins
Our plugins creates you log files in the Logs folder for the output: StdErrDataAPI.log and StdOutDataAPI.log. This way you can watch messages written out via our Trace function in scripts run via Data API.
Please try and don't hesitate to contact us with questions.
PS: This was actually asked to Claris via Ideas section: Support plugins in Data API and older idea Support plugins in Data API.