How do we design our own forms?

I have a client whose host may not be easy to work with on the “whitelist your domain for ModSecurity” issue. Also, I’d like to have more control over form layout and styling within the context of the WordPress site I’m designing.

I’d like my form to interact with the Constant Contact API just as a Form Designer form would. What do I need to know to make this work? I assume I need to assign specific names and classes to my form elements. Is there a list of these anywhere? Is it possible to design the form at the page editing level (working in HTML mode in a page editing window), or would I need to put my form in a page template (probably due to the need to call a php function)?

Is there a how-to on this anywhere? Or at the very least, is there any kind of reference to the names and classes I’d need to give to my form elements?

Thanks in advance to any and all help and response.
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Obbie