Hi Tiffany!
If you have HTML that was generated from an external source then one of the easiest ways to add it to your page is to switch the page editing mode from Visual to Text. Changing to Text mode displays all of the HTML markup for the page content and should let you add your form code. One thing to keep in mind though, WordPress has a number of text sanitization features that activate when the editing mode is switched back to Visual and you will likely need to keep that page in Text mode to preserve your form code.
Out of curiosity, what is the form that you are trying to add to your page? Sometimes there are integration plugins created by the WordPress community that can help with embedding external content, and it is possible that there is an available plugin that could make this process easier.
Please let us know if you have any other questions for us Tiffany!