if you have Enterprise with SA you can install Power BI Report Server on the same licensed server instead of the standard SSRS installation - PB RS is a superset of SSRS containing the same exact functionality as SSRS plus the capacity to host Power BI.
If you don't have Enterprise with SA then your only option is the service based Cloud option (quite expensive) which also allows you to have a on prem server with number of cores associated with the tier you "rent" on cloud.
Regarding your comment "like desktop on demand reporting" - PowerBI Desktop is that indeed - having PowerBI Report server is only to host it and be available for other users to use as "end users".
Note that those deploying/publish a pbix to any Power BI Server (being on prem or cloud) also require a license for Power BI Desktop pro - around 10 Euros per month.
For standard use on desktop there is no licensing requirements.