@Debasis Pati 

https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0694768

refer this article

Regards,

Poonkodi

@Debasis Pati 

it's OOTB behavior, internally it calls this method emailClientOpenPop

This method always picks 1st email client template created on your table

So you can't choose here

💡 If my response helped, please mark it as correct ✅ and close the thread 🔒— this helps future readers find the solution faster! 🙏

@Debasis Pati 

Thank you for marking my response as helpful.

💡 If my response helped, please mark it as correct ✅ and close the thread 🔒— this helps future readers find the solution faster! 🙏

One more thing i have a template created for my client and we use the template across all notifications.
in notifications we have a field called "Email template" and here i do set my template which contains the branding for my customer.

Now i have created some client templates for incident table in client templates i do not have "Email template " field so that i can call it how again i can apply the same branding here?