By convention, headers are set using Mixed-Case-With-Dashes. However, when they are referenced in substitution parameters, they are referenced in lowercase.
For example, a node might set a header and reference the same header directly like this:
Title: Some Example Node Foo-Bar-Baz-Quux: Some Value Here is the value of Foo-Bar-Baz-Quux: {{value:foo-bar-baz-quux}}
The example above would render in the generated page as:
Here is the value of Foo-Bar-Baz-Quux: Some Value