how to add html/javascript to post

You can create a Page Template for this page and apply that to the Page where you want the form to be. If the Javascript is external you can just use the providers code in your Page Template. If the Javascript needs to run from your site, review:

https://codex.www.ads-software.com/Using_Javascript

The CSS can be added to your stylesheet or you can create a header file to be used on this page and include the stylesheet(s) provided there.

Note, the styles may need to be tweaked for your site look.

All of the above should be done in a Child Theme.

Standard note on CSS:

To discover what CSS is output by your theme, use a web inspection tool such as Firebug: https://getfirebug.com/ , Chrome Inspect Element: https://developers.google.com/chrome-developer-tools/ or Internet Explorer 9+ F12 Dev. Tools: https://msdn.microsoft.com/en-us/library/dd565627(v=vs.85).aspx#csstool

There are others.

When editing CSS, use a Child Theme
https://codex.www.ads-software.com/Child_Themes
Custom CSS Plugin, or Theme provided custom CSS option.
Edits to parent themes are lost on theme update.

Learn CSS: https://www.w3schools.com/css/

thank you Pioneer! I’ll give it a try!

cheers