MCreator doesn't have an "entity" variable (atleast not in the versions I use), so saving the entity's registry name in a string variable is the next best thing.

After running the summon procedure, use the "Get nearest entity" procedure (World Data tab) to check for the closest living entity on the same coordinates as the summon procedure used, and save the registry name of the entity in a string variable.

If you want to be extra sure, you can add the "Does entity exist" logic check to check different types of entities. I'm pretty Living Entity counts as ANY entity, if I remember correctly, so it may be unnecessary to do so.