How can i add WYSIWYG editor for my custom form textarea?
Drupal 7 introduces the form element #type text_format, which is a text-format-enabled version of a textarea.

To integrate WYSIWYG into your custom form, just modify the attribute #type of form as:

'#type' =&gt; 'text_format'


Note: Tested it with CKEditor v 7.x.

In case if it doesn't work for you, please check your WYSIWYG configuration.

To hide format options, see: How to hide Input Format Options and help text under a textarea?

Above text field type works in Drupal 7 and 8.