How to add a text note to invoice?

I like to add some custom text on the invoice in 2 or 3 lines, to add some extra info.

I dont see a field for notes on the form for invoice.

How to add that?

You can use the “Remarks” field under the Accounting section
https://docs.erpnext.com/docs/user/manual/en/accounts/sales-invoice#320-accounting-details

or “Terms and Conditions” field which can be shown in the customer print as well
https://docs.erpnext.com/docs/user/manual/en/accounts/sales-invoice#315-terms-and-conditions

or you can create your own custom field using the customize form
https://docs.erpnext.com/docs/user/manual/en/customize-erpnext/custom-field