How to modify the subject line of Notification ? Where exactly to write the mail script for subject line as I want to change the subject line on basis of a field present on another table

[Servicenow12]

How to modify the subject line of Notification ?  I want to change  the subject line on basis of a field present on another table .

I referred the post below but not getting where exactly to call it 

https://community.servicenow.com/community?id=community_question&sys_id=4e1303e1dbd8dbc01dcaf3231f961966

Please see below screenshot I want the account field from 'x_nps_billing_table' to be copied to the subject line of notification.

Where exactly i have to mention the mail script in notification ??

Can somebody help with an example ???

Also please check the below script and suggest if something missed.

[Saurav11]

Yes after you have written the code syntax which i mentined in the email script you will call it using the above syntax in the notification in the message html Thanks.

[SAI VENKATESH]

Hi 

After adding the code in the mail script as said by Saurav you need to call it in the respective notification as 

${mail_script:name of mail script}

Thanks and Regards

Sure Sai Venkatesh