Dear Mark,
Good day!! Thanks for posting your concern in this Microsoft Community.
As per the description shared, I understand your concern and to my knowledge & search Microsoft Designer is a web-based graphic design tool that allows users to create various types of designs, such as social media posts, posters, and presentations. While it doesn't have an API specifically designed for integration with external applications, it does offer a robust set of export options that can be used to integrate with other tools.
In Microsoft Designer, you can export your designs in various formats such as PNG, SVG, PDF, and PowerPoint. This means that you can use these files in other applications such as Adobe Photoshop or Illustrator, or even in your own custom-built applications.
To get more information on this, I would like to suggest you to post your concern in the related community i.e., Questions - Microsoft Q&A to get the detailed help from experts.
Thanks for your understanding and patience.
Sincerely,
Mia | Microsoft Moderator