A. 
1) Setup an external billing product for selling the credits.
2) Add a billing option to the product.
3) Edit the Notify URL for the product putting your URL as shown below:
~wmtsystemurl~/admin/smsapi.php?memberid=~affid~&credits=#&apikey=XXXX###
# = Replace # with the amount of credits to add when payment is made
XXXX = Put your API Key from Advanced Features.
yourURL.com = Put your domain name.
4) You can then get the URL or ~~ code to use when when editing billing for placing the button throughout your site.
A. YES, it is possible to set it up using our API so that when someone joins the 3rd party system an account is created in the WMT Software.
If this is possible depends on the 3rd party software.
It would need to be modified so that when someone joins it, it calls the WMT API requesting that a new account is created.
A. 
1- Login to your Stripe account
2- Click API on left menu
3- Go to Webhook page then click Add endpoint button
4- Create an endpoint with the following values:
URL: ~wmtsiteurl~/admin/stripenotify.php
Mode: Live
Send me all events (selected)
Then click Add endpoint button
5- Go back to API page and copy both Publishable Key and Secret Key and paste them to your profile/settings page
