In your PackageContents.xml file, be sure to set both
SeriesMin and
SeriesMax to "R25.0" and
Platform to "Civil3D" to ensure that it can be loaded and only loaded in Civil 3D 2025.
<RuntimeRequirements SeriesMin="R25.0" SeriesMax="R25.0" Platform="Civil3D" OS="Win64"/>
Note: The release date is 2025 in this example. Use appropriate Civil 3D versions as applicable.