I did try to update the field but it getting changed to default again on its own.

If you have access to global, your domain picker would need to be set to global and then update the domain field. If you don't have access to global, you cannot create a user in global nor change the domain.

I have access to global domain and while creating the user  also global domain was selected but still it is considering the default domain. Even while changing the domain of a user I am working from global domain only. Don't know where are things going wrong..

Hi,

"If no domain is set as Default, Tasks and Users with no domain are placed in the global domain."

Thus simply open TOP/MSP/Default domain and uncheck the "Default" field on the form.

That's all.

With best regards, Dzmitry

@s.karpe   if you or others need to know the answer to this what you have to do is create the user in your normal method then back to the user record form or list ensuring the 'Managed Domain' & Domain fields/columns are on show and set 'managed Domain' from 'false' to 'True'.  Now you can change the  domain to 'global' or whatever you want. 

Don't change back to 'False' though as the domain will revert back to what is was before