Autodesk does not currently provide any data connectors for Power BI which would enable you to get data into Power BI directly from a given Forge service. Without that, moving data from Forge to Power BI has 3 steps:
1) Get data out of Forge
2) Move data into a database or file format that Power BI supports
3) Connect Power BI to the data source
The only Forge related part in the above process is getting data out of Forge, and that will vary based on what data you need.
You can get object properties from a model through the Viewer or through the Model Derivative API, there are API's to get issues and other data out of BIM 360, etc
So you just have to look at the specific Forge API's to get the data you need.
You may even decide to develop a Power BI data connector for the given Forge service 😃