Easiest way is to create an eventtypes.conf in $SPLUNK_HOME/etc/apps/TA-alert_manager/local
and update the eventtypes used by the app:
search = index=yourindex sourcetype=alert_metadata
search = index=yourindex sourcetype=alert_results
search = index=yourindex sourcetype=incident_change
For certification reasons, the index constraint hat to be removed. However, I think this is still a big need and will introduce it in a future version again.