how to manually fire up the plugin

Hi, I have a couple of years’ old website, back then there were no plugins to generate an image with a watermark so my developer coded one from scratch. The way it works is when you enter a product page, the script checkes for image with watermark and either displays it if it exists or generates a new image with watermark and shows it. Problem is, now I want to have webp images on the website amd I love to use your plugin to achieve this, but it fires up earlier then my script on entering the page and my watermark image is never generated. I would like to be able to specify when your plugin launches, and launch it after my script, so that it converts an image with the watermark already present. Can this be achieved?