1. For Blogger users:
>>> Open your blog by signing in your gmail account in which your blog is associated.
>>> after opening your website move your cursor towards the tool icon as shown in picture.
>>> You will find the widget id in the bottom of the page as shown in above picture.
2. For other platform users:
>>> Press ctrl+shift+i to open inspect element tool box or i.e. developer tools.Here I am using in Google Chrome.
>>> Click in the inspect icon as shown below.
>>> After clicking on your widget you can see your widget id in the inspect element tool as shown below.
This is a simple tutorial to show you how to find out widget id on any widget in blogger or any other platform.