How do I add a custom field token to a default value?
I know this was asked years ago, but if someone comes across this, here's the solution:

https://www.drupal.org/node/919760#comment-9973163

Node tokens in a Webform block refer to the Webform node, not the current node which the block is placed on. Currently, the Token module does not expose node tokens to the Webform block.

There is a patch in this thread which exposes tokens to the Webform block, and should do just what you need. Hopefully it will get committed to the Token module soon. :-)

As an alternative, you could also use https://www.drupal.org/project/token_custom and create a custom token that exposes the field you need.