Hey @amit1, the last name field is required by default when creating meetings through API and enabling the registration required.
For the webinar, the default value would follow the web settings, please navigate to Account Settings > Webinar Settings > Registration Settings, please the below screenshot.
For the issue, can you please set the default settings in Webinar Settings for the webinar, but for the meeting, You can disable it through the web portal, or call the Update meeting registration question API: 
Could you pass some dummy values for the last name field via API and check if it’s working?
There was a bug fix to enforce this as it wasn’t being enforced before. You can refer to this 👇
Hope this helps!
