How do I set the HTML5 date "min" attribute? I have a date field set up on a node in Drupal 8 using the default date time field formatter. I would like the date picker pop-up to start with a specific date I have defined in a config variable.

How can I do this? I have tried the code suggested on How to add select years and months select Drupal 8.1 date picker? but it doesn't do anything for me. 

I have also looked for a way to pass datepicker options to the field element (looks like you could do that in Drupal 7 date module), but couldn't find a way to do that either.
In Drupal 8, the datetime fields are HTML5 inputs, so the popup is provided by the browser itself, or from a polyfill library for browsers that don't support these inputs (eg, Firefox).

If you want to provide a default value, you can do it from the settings for the field.  You can use the Current Date or a Relative Date as the default.  Despite the name and the docs on Relative Date, you can use this to set an actual date/time.

If you need to pull the default value from somewhere else (like config), you would hook_form_alter() the node form, and then check the field value.  If it is empty, you would set the value there.