How to change Parsing Schema on runtime in Oracle APEX

  first make select list item  P4_AUTHORIZTION_SCHEMA

that will contain all DB schema available for your workspace

use this code as source 

select APEX_200200.wwv_flow_escape.html(schema) d, schema r

from   APEX_200200.wwv_flow_company_schemas

where  security_group_id = :flow_security_group_id

order by 1

and make process to update Application Authentication 

UPDATE APEX_200200.wwv_flows 

SET SECURITY_SCHEME =:P4_AUTHORIZTION_SCHEMA

where ID = :APP_ID;

--
thanks