You can use the application item as bind-variable with its name. ie. your application item is named G_MY_APPLICATION_ITEM, then you can access/set it using :G_MY_APPLICATION_ITEM.
-- assign like a variable
:G_MY_APPLICATION_ITEM := 'LARRY';
-- use in a SQL statement
WHERE USERNAME = :APP_USER
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If you want to set the item value when user start a session,you may put the code in Home Page loading.