Hello @DonS3096 ,
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API. Typically we work directly with those developers on issues surrounding creating integrations and interfacing with our API.
When looking into WP Form's documentation further it looks like the authorization code is generated when you click on the "Click here to register with Constant Contact" link. Once authorizing WP Forms to connect to your Constant Contact account then you're prompted with the authorization code. Below is a link to their documentation which supplies the steps on how to connect WP Forms to Constant Contact and how to obtain that authorization code.
How to Connect Constant Contact With WPForms
Please reply directly to this post if you have any other questions!