I try to make a modal form

Hi Kittisak

this is outdated for v8, it's from 2015

You can use a Joomla modal extension to load the form, OR

Use a workaround to load the built in modal feature in the Chronoforms8 NUI JavaScript library:

Add this code in your Joomla article:

<a href="index.php?option=com_chronoforms8&chronoform=form-alias" data-modal="">Click to open</a>
{chronoforms8}empty-form-alias{/chronoforms8}

This will show a link and when you click it you will open a modal with the form with alias "form-alias"

There is a call to another "empty-form-alias", this is needed in order to load the javascript library on this page, this form can be completely blank, it's called to load the JS library only