Hi Chris,
The JS-API is considered part of the Internals of @anywhere and isn't supported by Twitter. You are free to experiment with it but please be aware that features and functionality may change or be removed without notice.
With regards to what you are trying to achieve, the TweetBox supports the status update parameters.
You set these parameters by using the 'data' object for the TweetBox. You can put any key: value into the data literal which is supported by the REST API.
Here's an example using the data literal to set the 'in_reply_to_status_id'.
twttr.anywhere(function (T) {
T("#example-metadata").tweetBox({
'defaultContent' : "@anywhere thanks!",
'data' : {
'in_reply_to_status_id' : '17889110158'
}
});
});