Hello,
In that case you will need to create a custom inline control that you can insert using the Insert inline control button on the
WYSIWYG editor toolbar, as the Insert BizForm button inserts the default BizForm inline control.
This custom inline control could contain the BizForm control with the OnAfterSave handler just as your custom BizForm web part does.
For more information about developing custom inline controls in Kentico
CMS R2, I would like to point you to the following article:
How to develop inline controls.
Best regards,
Michal Legen