As Custom Fields for Line Items can be created for Sales Invoices (Revenue), why not also for Receipts & Payments (Cost). Then if necessary, cost transactions can be matched with revenue transactions, for example with a specific tagging. This can be helpful for companies to use in reports
Did you try, i.e. download the latest version, create a custom field (like text) and select Journal entry? It does not apply even though one can select it to line items but it does apply to the Journal entry itself.
Please read the guides https://www.manager.io/guides/9820 why for payments and receipts (which you should enable) are not part of Journal entries.
In fact the guides state for example
In Manager, most transactions are entered in other functional tabs, thereby automating many decisions about account posting and reducing errors. So journal entries are relatively few. Most record transfers between accounts. In fact, no transaction involving the actual receipt or payment of funds by a business can be recorded via a journal entry.