Firstly, you should not be trying to set headers that Asterisk manages using add header features. What are you really trying to do here? If you cannot do it by, for example, specifying a proxy, it is probably not safe to do it at all.
Whilst there are obvious syntax errors, because of the > and ) being commented out, I don’t really understand why you are getting a complaint about their being no equals, as Asterisk tends to and missing final )'s and the dialplan doesn’t no the valid syntax for the header value.
If this weren’t a core header, I’d suggest escaping the ; with a \ but, as it is a core header, I’d suggest trying to use it by built-in methods.