Prefilling or pre-populating Grav Form fields is not an out of the box thing.
To make the use of dynamic fields and function calls (see documentation) easier I’ve created a new plugin, the Form Prefiller Plugin.
It is a first release and thus will not yet show up via GPM. Therefor you need to manually download and install te plugin for now.
Issues may be reported in the GitHub repository.
Please let me know whether you think it’s useful or not.