Postgresql create index concurrently
CREATE INDEX CONCURRENTLY cannot run inside a transaction, and all functions are transactions, (but ordinary create index can).

Perhaps something like PG_AGENT can be leveraged to create the index upon command from your trigger.

https://www.pgadmin.org/docs/pgadmin4/3.x/pgagent.html

Alternatively you could do a regular create index when you create the table, an empty table will not be locked for long during a create index.