You can use the 123FormBuilder Publish → Embed Code section. Next, in your Yahoo SiteBuilder project, open the webpage where you want to include the form. From the toolbar on top, go to Insert → Code Elements and select HTML.
Note that the online form is not visible in the editor, so you’ll need to publish the page in order to see the results online. Adjust the widget on the web page as you see fit and you are done. Visitors can now submit your online form directly on your Yahoo website.
In making this documentation, we have used the version 2.8.7 of the Yahoo SiteBuilder.