Autopopulate form fields on an remote domain

I need some code that will allow me to use data on a website that my company hosts and controls, to populate forms on a remote website over which we have no influence. The owners of the remote site are not interested in assisting, so there is a problem with cross-site script, or Same Origin Policy. In order to populate the remote form manually, the user must log on and navigate to the correct page. The ideal solution will be using JavaScript, perhaps using a JSON/JSONP object as the data source. The user will open a client record on our site, and see the remote form below the client's details, perhaps in an iFrame. There will be a button on OUR form that will cause the relevant data to populate the correct fields on the remote form. They can then look it over and click the submit button that is on the remote form. If this is not achievable, and it must be a stand-alone application, it should be compatible with the Windows OS. Our server is running IBM's Lotus Domino. IMPORTANT POINTS TO NOTE: 1. We design and host my client's website, which holds details on their clients. This is validated data, held in a database. 2. A client record is the source data that must be used to populate fields on the remote form. 3. The remote site requires the user to log on, and uses HTTPS. 4. There is more than one page on the remote form, so the user will need to be able to click the submit button, ON THE REMOTE FORM, in order to get to subsequent pages. 5. The end result will be that the remote site will generate a reference number, which the user will copy into the client form ON OUR SITE. So simply submitting the data to a URL will not be sufficient. 6. The remote form is NOT in our control. 7. If this cannot be done as a browser based solution, the application must be able to communicate with a Lotus Domino server to retrieve client records based on unique references, for use as described above. The objective here is simply to remove the possibility of human error. The data on my client's site has been validated, and at the moment must be painstakingly transcribed either from paper or using copy and paste from a different page. This is not acceptable, due to the volume of work. If this is not sufficiently clear, please let me know and I will clarify further.