This is a very usefull tool, is it possible to do the same with Libreoffice Writer from navigator tab or other tool?
Edit your question because I don’t recognise the dialog (icons and items) in the screenshot. Which application is this? If Writer, what kind of UI have you chosen?
Also, pay attention to vocabulary: your title asks about headers while the screenshot is about headings. These are two different things.
In all cases, mention your OS and LO version.
Header and Footer are terms only applying to PageStyle.
What you are talking about obviously should read “Heading” in all the places.
Please try to also use terms the way they are used by the UserInterface. We may get misunderstandings or even a complete mess in terms otherwise.
If you urgently want to use a different (shorter e.g.) term for specific reasons, please introduce it explicitly first.
Headings of different levels are managed based on the Paragraph StyleFamily in LibreOffice, and a term “Subheading” is not usual. You generally don’t add headings using a specific tool, but create them as ordinary single paragraphs first, and then assign the respective ParagraphStyle from the hierarchical group Heading using Manage Styles (F11) (the former “Stylist”).
@ ajlittoz: I suppose, the image is from some alien (hostile?) software.