Your Apple ID and phone number are now being used for iMessages on a new iPad

If you recently set up a new ipad or if you have logged in on a fairly new iPad or if you regognize the device, click OK

If you don'; recognize the login attempt please see this article If you think your Apple ID has been compromised - Apple Support

Also check your device list to see if there is a device signed in that you don't recognize. You can remove the device using the steps in the article below. As a precaution, change your password.

Apple ID device list -Check to see where you‘re signed in.   * 

 Apple ID Password Change.   -  

ASC YouTube for How to reset your Apple ID password

 What to do after you change your Apple ID or password - Apple Support  -   I suggest doing this before you change your Apple ID or password.