If you have an existing form then it can be really useful to be able to copy and paste the HTML directly into ChronoForms. There are two ways to do this.
The simplest is to copy and paste the HTML into the Code box on the Form Code tab (click the Name link in the Forms Manager to access this). Then set the Form Type to 'Custom' to stop the code being overwritten.
You can then use the Form Wizard to add actions to and configure your form.
The more flexible (and in my opinion better) approach is to use the Form Wizard and drag a Custom Element element from the Advanced Group into the Preview window. You can open this action and paste your code there.
The result is the same except that using the Custom Element element means that you can - for example - very easily remove the Submit button from your custom HTML and drag in ChronoForms Captcha and Submit button elements to work with your code. This ability to 'mix and match' can be very useful.
Note: please remove any <form . . . > and </form> tags from any custom HTML or your form will probably not work correctly.
Note: please remove any <html> & </html>, <head>. . .</head>, and <body> & </body> tags from your HTML including all the contents of the <head> tags. Your HTML will be included in the body of a Joomla! page and including these tags will probably break the page.