Set Form fields defaults from plugin

I have a simple front end contact form and added another field for text input. The form is completely defined in the frontmatter and operates as shown in the grav documentation. I want to use a plugin to prefill that added field dynamically to a variety of things. I am able in the form definition to set a “default” value, but that is not dynamic. In my plugin, what event would I use to get access to the form field parameters and re-set the default value before the form is displayed? So a more general question I guess: How to use a plugin to set default values of form fields before form is displayed?

The Form plugin does not currently provide such feature. Open an issue in with the details of how that should work.