You can use the plugin cordova-plugin-device to get a UUID from the device ( this api is the same on iOS,Android,... )
This is NOT the exact same thing as an IMEI but it is sufficiently unique to identify the device, while still protecting the privacy of the phone owner.
If you must get the IMEI, then google is your friend. This  was the first result when searching for 'cordova imei'
There is a good stack overflow article here  which outlines the downside of using this value, and urges you to consider other ways of identifying users, because users may not trust your app if you request this info. Read the accepted answer.