Drupal 8 Webform - Pass submitted value from one webforms to another
Ok I figured it out - for anyone else who comes across this question. 

In the first Webform, I set the Confirmation URL to:

/form/my-form2?value=[webform_submission:values:element_key]

I've bolded the elements in the URL that you will need to adjust for your webforms. 

This will result in redirecting the user from my first Webform to the second Webform (which is at the URL www.mydomain.com/form/my-form2) when they click the submit button. It will append ?value=[the value from the webform field] to the end of the URL. 

Then in the second form in the field where I want this value to go, I have set the Default value for this field as 

[current-page:query:value]

The word 'value' in the above can be anything, but it must match the id used in the URL from the first form. 

And hey presto - it works!

F