org-html-preamble

org-html-preamble is a variable defined in `ox-html.el'.
Its value is

t

Documentation:
Non-nil means insert a preamble in HTML export.

When t, insert a string as defined by the formatting string in
`org-html-preamble-format'. When set to a string, use this
formatting string instead (see `org-html-postamble-format' for an
example of such a formatting string).

When set to a function, apply this function and insert the
returned string. The function takes the property list of export
options as its only argument.

Setting :html-preamble in publishing projects will take
precedence over this variable.

You can customize this variable.