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.