How to use form information to auto populate a form on another webpage

Question

I am trying to figure out a way that I can populate form fields on webpage for my users much like password managers do. The problem is that I am not the owner of the second webpage. I thought about using javascript with iframes but that doesn't work. I've tried using php to replace the form information adding values saved from my previous form. But I need to add info on a second form after the first one is submitted. After I submit the first page I am off my page and can't change anything else. So I'm kinda out of ideas and my knowledge is limited. Any ideas or input would be greatly appreciated. Thank you for your time and effort.

Answer 1

Cannot be done because of the SOP (same origin policy) enforced on JavaScript code. An alternative would be through XSS, other via a bookmarklet, and as a last choice trough a GreaseMonkey script.

GreaseMonkey may be your best choice, if the data to be filled in is from your website. Best choice because GreaseMonkey scripts can perform cross domain ajax requests.