What do you mean?
The trigger_ entity, or the entity you want the trigger_ entity to trigger?
If you have an entity, such as env_explosion, you can trigger the env_explosion by giving it a name such as "explosion01" and then make a brush based trigger entity such as trigger_once, and in the properties of trigger once, you look up the field "target", in that field, write the name of the entity you wish to activate, in this case explosion01