multi line header text for custom page

I've defined a custom page for my installed and need to have a multiline header. Can someone point me in the right direction for this?

Here's the relevant sections of my code:

Page custom custom_PAGE ;Custom page



Function custom_PAGE

!insertmacro MUI_HEADER_TEXT "$(SCR_EXISTS_MSG_TITLE)" ""



nsDialogs::Create 1018

Pop $Dialog

nsDialogs::Show



FunctionEnd