You can create your custom field for the invoices here.
Go to Settings > General > Custom Field Details tab.
How to define/add a field in Invoice/Quote/Retail / Invoice/Receipt/Purchase Order
- Go to Settings > Sales > Custom Field Details tab
- Click on Add Item Button. In the field details popup, Select the type of invoice on which you wish to add.
- Provide the field name (required) and value, if any. If you wish to have this field as required in the entry, enable the“Required field in Invoice entries” option.
- To print this field in the selected invoice type, check/enable “Print in Invoice”.
- Enable the Active option to use the new/desired field in transaction.
How to edit/delete a custom field entered?
To edit/delete a custom field,
Choose the Edit/Delete option from the drop down associated with each custom field.