Beatrice Avenue wrote:

To create a new custom field navigate to -> Administration -> Custom fields and select the section for which you will create a new custom field, e.g. for work packages.

You will see the list of all the custom fields that have been created so far, divided by the section on the different tabs for which they were created.

If none have been created so far, click on the link Create a new custom field in the respective section, e.g. on the tab for work packages.

This does not answer the question asked about installation and PostgreSQL: choosing a schema other than public.