Add To Segment Step
Add to segment works with the current lead of the run. Therefore, it is available only to For dialogs automations and does not appear in the catalog of a background workflow.
Add An Action
Choose the required step from the catalog. Then select the added step on the workflow, choose a segment in the settings panel, and save the step.

The step submits a subscription request for the current lead. If the segment requires consent confirmation, the result depends on consent settings and may wait for confirmation.
Account For Repeated Starts
Adding a lead may become an event for other automations with an Entered segment trigger. The same automation does not start itself again from the event it created, but you should still review connected workflows for a possible chain.
What Is Validated Before Execution
A segment is required. Publication is blocked if the selected segment is missing, archived, or belongs to another project.