The default logon option in OWA is using the format : Domain\username  as shown in the below snapshot

You can change the default logon format to one of the following formats:

• [email protected]
  
  
  

• Username
  
  
  
  
  

So lets check how to change the OWA logon format in Exchange Server 2013 :

1. Login into Exchange Admin Center and then from the left side menu, click on servers
   
   
   
   
   
2. Click on virtual directories
   
   
   
   
   
   
3. From the list of the available virtual directories, click on the owa (Default Web Site) and then click on the edit button 
   
   
   
   
   
4. The owa virtual directory options page will open, from the left side menu, click on authentication
   
   
   
   
   
5. As  you can see, three forms-based authentication logon formats are available
   
   Domain\user name
   User prinicipal name (UPN)
   User name only
   
   
   
   The default option is the Domain\user name , to set it to logging in using your complete email ID , then select User principal name (IPN) and click on save
   
   
   
   
   To apply the changes, you must restart IIS .
   
   Note : Check the steps at the end of this article to know how to restart IIS.
   
   
   
   
6. After IIS is restarted, the logon format will be as shown below. My email ID is [email protected] so I will login into OWA using tarek.maj[email protected] instead of elmajdal\tarek.majdalani
   
   
   
   
   
7. To Set the logon format to username only, select User name only
   
   
   
   Then select the domain name by clicking on browse... and from the list of available domains, select the domain you want to authenticate your users from then click ok
   
   
   The domain name will be selected, click save to set the forms-based authentication format option to be username only.
   
   
   
   
   Of course, you must restart IIS to apply the changes
   
   
   
   
   After IIS is restarted, the next time you try to logon, you will be presented with the username only logon format as shown below
   
   
   

To restart IIS :

• Open IIS
  
  
  
  
• Then from the left side pane, click on the server name and then click Restart from the right side pane