You can change the parsing schema using the apex_util.set_parsing_schema_for_request procedure from within the application’s initialization code. Follow these steps:

Click on the Shared Components > Security Attributes > Database Session.

Then add the following PL/SQL code in the Initialization PL/SQL code section:

Declare
   v_parsing_schema varchar2(30);
Begin
   Select user_schema into v_parsing_schema
     from your_user_table
     where username = :APP_USER;

apex_util.set_parsing_schema_for_request (
   p_schema => v_parsing_schema);
End;

Change the table and column name according to your table.

Below is the screenshot for reference: