To create a new Azure AD directory named 28681041.onmicrosoft.com that contains a new user named user1@28681041.onmicrosoft.com, you can follow these steps:

In the Azure portal, search for and select Azure Active Directory.

In the left pane, select Domains.

Select Add domain.

In the Add a custom domain pane, enter the following information:

Domain name: Enter the domain name you want to use. For example, 28681041.onmicrosoft.com.

Add domain: Select Add domain.

In the left pane, select Users.

Select New user.

In the New user pane, enter the following information:

User name: Enter the user name you want to use. For example, user1@28681041.onmicrosoft.com.

Name: Enter the name of the user.

Password: Enter a password for the user.

Groups: Select the groups you want the user to be a member of.

Select Create.

You can find more information on these topics in the following Microsoft documentation:

Add a custom domain name to Azure Active Directory

Create a new user in your organization - Azure Active Directory