If you need to account for project manager's time spent, well, managing the project -- as opposed to performing project tasks -- you can use the Forecast Project Leadership feature.  Same thing if a project will require X% of a senior manager's time.

From the Project 360 page, go to Project Settings then Resources.  Then select the resource , the primary project role will pre-populate if available, and then enter the estimated percentage of their overall time will be allocated in this role.  Two key points:

• Project leadership is forecasted in terms of percentage only for convenience and practicality purposes.  It is applied for the duration of the project.
• You can override the project role for instances where, for example, a senior developer is assigned tasks for writing code, but they also happen to be wearing a project manager's hat.  The estimated effort for both roles are included in the senior developer's overall forecasted workload.