You can set the TrustOnly flag for a trusted user by specifying the WITH TRUST_ONLY option with a GRANT CONNECT THROUGH request (see Teradata Vantage™ - SQL Data Control Language, B035-1149 for the syntax and rules for using GRANT CONNECT THROUGH requests).
When you either commit or roll back a transaction, Teradata Database removes both its query band and any proxy user who has been set with a query band for that transaction. Because you cannot control this by setting the TrustOnly flag, you should instead set a session query band to set the TrustOnly flag when you require it to be in effect for longer than a single transaction.