I want to modify the contents of a field of an Article node just before it comes up for editing. I have read the documentation at http://api.hostdog.gr/api/drupal-8/drupal%21core%21modules%21node%21node.api.php/function/hook_prepare/8, which states that hook_prepare will "Act on a node object about to be shown on the add/edit form." I've read lots of other stuff too.
I've tried lots of ways of naming the function:
MyModule_article_prepare() MyModule_article_node_prepare() MyModule_prepare_article() MyModule_prepare_node_article() and on and on
But the function is never called. I've tried putting these functions in both the MyModule.module file and in the MyTheme.theme file. I've read countless posts. Nothing has caused my function to be invoked (I output to the error log to tell it got called).
I simply want to look at one field in the node and possibly modify it just before it comes up in the add/edit form for the user to edit it. I feel really dumb; this has got to be dead simple and I'm just not getting it.