To integrate Authorize.net payment gateway, please go to OptimizeMember >> Other Gateways, check the box for "authorize.net" and save the settings.
Then, go to OptimizeMember >> Auth.net options and go through the fields.
Authorize.net uses pro-forms only. To generate pro-forms, please go to your OptimizeMember >> Auth.Net Pro-Forms >> choose the form you want to use and configure >> Generate the Form Code >> Copy and Paste it into your Posts/Pages.
Authorize.Net Pro-Forms integrate seamlessly with WordPress Shortcodes.