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]

Hello

You habe to use the beliw syntax

email.setSubject(+ gr.u_account); instead of template.print

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[Servicenow12]

But where i should call it in notification ? whats the syntax ?

[Saurav11]

To call it, In the message html right the below syntax

${mail_script:mailscriptname}

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[Servicenow12]

I have to call this to modify subject Line ....still i will use this syntax ?