Business Rule: Capture logged-in user ID?

Summary:

Is there a way to capture, say in a variable, the ID of the logged-in user?

Content (please ensure you mask any confidential information):

We're building a custom, multi-level approval process (the inbuilt processes don't suit the client's needs). When approving an item (employee, CapEx item, etc.) we'd like to have the system auto-populate the ID of the approver, as opposed to selecting themselves from a Smart List.

Version (include the version you are using, if applicable):

Current EPM Planning Cloud

Code Snippet (add any code snippets that support your topic, if applicable):