I might be bit late on this post, but has same requirement so:

@arnabbhaumik  You can add subject in your code by getting the ticket number and short description of recrd and then pass the same in url.

 

<?xml version="1.0" encoding="utf-8" ?>
<j:jelly trim="false" xmlns:j="jelly:core" xmlns:g="glide" xmlns:j2="null" xmlns:g2="null">

<a class="btn-default;" id="${jvar_n}" onclick="userWindow('${ref}');">
<img src="teams.jpg" width="25%" height="25%" title="Teams Chat" alt="${gs.getMessage('Click to open Teams Chat')}"/>
</a>

<script>

function userWindow(reference){

var prefix = 'https://teams.microsoft.com/l/chat/0/0?users=';

var refVal = reference.split('.');
var fieldName = refVal[1];
var emailId;
var fName;
var subject;

var sysId = g_form.getValue(fieldName);

var usr = new GlideRecord('sys_user');
if(usr.get(sysId)){
emailId = usr.email.toString();
fName= usr.first_name.toString();
subject = '&amp;amp;message=Hi ' + fName + ', this is regarding your Case ' + g_form.getValue('number') + ': ' + g_form.getValue('short_description');
//g_form.addInfoMessage(emailId+' '+fName+' '+'Subject: '+subject);

}

if(emailId){
var w = getTopWindow();
var url = prefix + emailId + subject;
w.open(url);
//w.open(url,'_blank');
//open('sip:'+emailId, '_top');
}else{
g_form.addErrorMessage('User's email is empty');
}

}

</script>

</j:jelly>

 

 

Please mark helpful if this fulfills your requirement.