The easiest way of configuring projections for your WebOffice 10.8 SP2 project is by using the VertiGIS Projection and Geotransformation Helper. You can either access it directly online under https://resources.weboffice.vertigis.com/Util/projection_helper.html or via WebOffice author standalone.
First enter either the name of the source coordinate system or the respective EPSG code, e.g. 3857, then do the same for the target coordinate system. If you are unsure about the projection of your project, you can:
•check the EPSG code of your main map service in SynAdmin
•open the respective MXD document and check the data frame properties
If the used geodetic datum differs between the two defined coordinate system, you will need to further define a geographic transformation. VertiGIS Projection and Geotransformation Helper will automatically provide the list of available projections.
Now configure the projection in your WebOffice 10.8 SP2 project by using the EPSG codes and defined settings.
Note: For custom projection and custom transformation (no EPSG and geographic transformation code available) you can insert the full strings with parameters. Navigate to your custom PRJ-file and open it in a text editor. Copy the content and paste it to Projection ID/String. Then open the GTF-file and copy the readable content (not the binary content!) to Datum ID/String.
Note: For using custom projections it is strongly recommended to define custom transformations. This can be done by using the Create Custom Geographic Transformation Tool from the ArcToolbox. The transformations can be defined and the GTF-Files are saved by default in <AppData>\Roaming\Esri\ArcToolbox\CustomTransformations.
Note: Find more details in chapter List of Projections.