When using the Matomo Connector for Looker Studio, there are multiple ways to apply a segment.
Configure reports with a segment by default
If your report is primarily focused on a specific segment, you can configure it at the time of connection. To apply a segment to your imported report, select the segment from the drop down menu when configuring the Matomo connector.
Apply segments using Looker Studio properties
Alternatively, if you selected to Allow “Segment” to be modified in reports. when configuring the Matomo Connector you can set it per chart. While viewing your report in Edit mode, click on a chart to load the Properties panel. There you will be able to set which segments data is included for that chart.
If you can’t see the Segment option within your Properties panel then you may need to edit the connection settings for the Matomo data source, or reconnect with updated settings.