Hi Medforce Team!
I will be happy to help you with this project.
The Add-on menu will be populated with the three options as you asked, the first one will be used to enter the user credentials that will be used to connect to the API. The credentials will be stored in the properties service using the user scope.
The second menu option will show the send button to the user as you illustrated and we can disable the send button if the user hasn't saved his credentials with showing the warning message.
When the send button is clicked, the script will make a copy of the current as pdf file, then make a POST request to the API with specifying the content type as multipart/form-data.
After the API call is successfully performed, we can either show a button to open the createSubmissionPageUrl in a new tab or we can show this page as an iframe inside the Add-on sidebar.
The Help form will be as exactly as you described.
Please have a look at my reviews, I am well experienced on Google Apps Scripting, have worked with numerous APIs, written many standalone scripts, Docs, Sheets amd Gmail Add-ons, and Google Script web apps.
Examples:
1. Recently, I have worked on a Doc Addon that allow the user to generate csv files ready to be uploaded to UPS to create UPS shipments. The Add-on also populated the current Doc to be the quote file sent the user.
2. Gmail Add-on using that allow saving the current message as pdf file to BOX.
I will be more than happy to address any questions!