How to get Sales Invoice ID

Im trying to create a new Print-Layout but im stuck at getting the Invoice ID.
The InvoiceNumber was originally in the Headsection of the Layout.
But iam using Custom Code in the Headsection so the Default

Sales Invoice
SINV-0002

gets overwritten

How do i get the Invoice ID as doc.naming_series gets me only the Series and doc.title gets me the Client the Invoice is issued to.

To show the Form ID try with code {{ doc.name }}
Also refer following link -

Thank you very much.

Just found it that its also mentioned in the footer note of the Custom HTML Field

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