Question   If I provide the API key to a third party affiliate program, will a new member signup in that program automatically create an account in my site for that member?

A. The name of the you is:
http://YOURDOMAIN.COM/join

Your can edit the template file for it at:
http://YOURDOMAIN.COM/uploads/adminpages/join.html

A. With PayPal, after a person pays, they are required to click a button (that is not noticable to many people) to continue back to your site to finish the join process.  This can cause many people to get stuck in the PROCESSING mode.  With PayPal, you can have it to automatically redirect to your website after payment so the person is not required to click on a button to proceed.  To do this, you must turn this on within your PayPal account.

A. An application programming interface, or API, allows other websites to communicate with our website sending commands to add/edit/delete items and getting reports.

These are used by developers to integrate 3rd party software with our software.  For example, our API allows you to have members automatically created in your system when someone signs up at a separate site you own.

A. In your members area, go to the "Advanced Features" page, then click on the "Misc" tab, then put the url for your webinar page in the "Redirect Page" field.
Put the full url, including http://. 
Also, put ?user=~username~ at the end of the url so that it will go to the appropriate member's page.

Exa:
http://yourdomain.com/webinar.php?user=~username~

A. Yes, it's editable by editing the .html file in the File Manager. 

The file you would edit is:
/uploads/adminpages/do-join.html