In the router configuration, it is possible to specify multiple servers for one shard. Then the router will distribute read-only queries among the replicas. However, in addition to the automatic routing, you also have the option to explicitly define the destination for a specific query by using the target-session-attr parameter within the query. See Routing hints for more information. 
-- you could specify target-session-attrs anywhere in your query
SELECT pg_is_in_recovery(), id FROM tsa_test WHERE id = 22 /* target-session-attrs: read-write */;
NOTICE: send query to shard(s) : sh1
 pg_is_in_recovery | id 
-------------------+----
 f                 | 22
(1 row)