Access Token for PHP mail form

I've read through the docs, but I am still confused as to how I get an access token in v3 to use on a simple PHP web form (newsletter signup). When I read the info, it sounds like a token is temporary, but I need this to be hard coded on my website. Also, the listed process is confusing.

I just want to add a new contact to a Constant Contact list when a user checks the newsletter signup checkbox on my website quote/contact forms.

I see the API Key. After that it starts to make less sense to me (Redirect URI, OAuth2). Any help would be greatly appreciated.