How to use handler: 'string' in joomla modals

Question

Im looking for some examples of how to use {handler: 'string' ...} for joomla modals, as used in editors-xtd buttons plugins

$button->options = '{handler: 'iframe' ......}';

but I want to use:

$button->modal = true;
$button->options = "{handler: 'string', content: 'some content'}";

but I haven't found any information for the proper usage of this or if this really works. I have spent many days trying to get it to work, I tried with several var names setting variables like $button->content or $button->html, or inside the $button->options, but nothing worked for me. Thanks in advance!