1. Create a subdirectory inside your joomla directory and Install the Universal Form Builder e.g. www.yourjoomla.com/Zigaformapp/
2. Create your form and set your options.
3. In order to enable custom script on editor text. just go to Extensions->Plugin manager->Editor TinyMCE. and remove “script” tag and save changes. 
4. Get the widget code from Zigaform backend application
5. Copy and paste to the page on joomla site. and thats all 

Solution 2:

just login to administrator then go to System–>Global configuration, under that you will see “Site Settings”, in that you will find-out “Default Editor”, select as “Editor – CodeMirror” and save it. after that go to articles, then add your script. Enjoy

Solution 3: 

it is using jce editor. you can download from here: https://www.joomlacontenteditor.net/

then you can add any other tag.

Solution 4:

just disable the Plugin Editor – TinyMCE temporarily, then you can add the widget code.