Send custom data to endpoint based on checkbox value via webforms POST handler Drupal 8.9, webforms 5.x

I'm trying to send some text data to a endpoint on form submission via a POST handler. It's based on the value of a (single) checkbox on a webform. The endpoint only accepts specific text values, so I need to somehow convert the 0/1 to those specific text values - if checked => "Signed up", else => "Not signed up".

I want the equivalent of

"key": [webform:elements:checkbox_machine_name:checked] ? "Signed up" : "Not signed up"


if you could have a ternary in the yaml format...

Is there a way I can do this via webforms?

I'm happy to install more modules, but if possible I'd like to avoid writing any PHP...
You could use a Computed Twig element to execute the ternary operator and set the value in the remote post data. 

@see https://www.youtube.com/watch?v=rRlTTkwdP_o