To define the Receiver Query on a Mailing with a "Custom Query - Other user must select receivers when sending the mailing" created through the Event Email Template module, you will need to have distributed the Mailing, so it is accessible in Communications > Email. Refer to How to create Event Email Templates (BB734903) for creating a new Event Email Template.
 
1. Access the instance of the Mailing in Communications > Email
2. Click on the blue "Receiver" button for this mailing
3. Choose "User must select receivers when sending the mailing" and click Save
4. Enter Receiver Query criteria on the Compose Query window that pops up
5. Click Done
6. Enter a desired Query Name and click Submit