How to add target=_blank to url?

Sorry for the delay. I’m a little confused — if your endpoint is https://somesite.com/submission, but you want to include a querystring, you just enter it as well in the submission URL field like https://somesite.com/submission?target=_blank.

Or do you mean that you need it to append it dynamically, from a form submission or mapping? Similar to this question about making a GET request, you could use the bypass hook and make your own post, customizing the url as needed (using functions like http_build_query, etc)