Fisrts step is to amend the mail template that is defined in a collection that is initiated in the app OnStart property:

If you amend that, you can include your personal preferences. Without testing, this is what I think it should look like:
ClearCollect(EmailTemplate,
"<html>
<head>
<meta http-equiv=""Content-Type"" content=""text/html; charset=us-ascii"">
</head>
<body>
<p>
There is a leave request from: {SubmitterName} pending your approval. <ul><li>Type: {LeaveType}</li><li>Title: {LeaveTitle}</li><li>Description: {LeaveDescript}</li><li>Start Date: {LeaveStart}</li><li>End Date: {LeaveEnd}</li><li>Requested Days: {RequestedDays}</li><li>Current Leave Balance: {CurrentLeaveBalance}</li><li>New Leave Balance: {NewLeaveBalance}</li>" &
"</p>
</body>
</html>"
)
Then you need to modify the mechanism that populates the mail template to include the new fields. This can be found in the OnVisible property of the ConfirmationScreen:

If you replace the code between the two comments with the below, it should work:
//create email template data specific to the request
ClearCollect(
TemplateData,
{
Field: "{SubmitterName}",
Data: _myProfile.DisplayName
},
{
Field: "{LeaveType}",
Data: _selectedLeaveType
},
{
Field: "{LeaveTitle}",
Data: TextInput1.Text
},
{
Field: "{LeaveDescript}",
Data: TextInput1_1.Text
},
{
Field: "{LeaveStart}",
Data: Text(
LeaveStartDatePicker.SelectedDate,
"[$-en-US]mmm. dd, yyyy"
)
},
{
Field: "{LeaveEnd}",
Data: Text(
LeaveEndDatePicker.SelectedDate,
"[$-en-US]mmm. dd, yyyy"
)
},
{
Field: "{RequestedDays}",
Data: _RequestedDays
},
{
Field: "{CurrentLeaveBalance}",
Data: Label1_12.Text
},
{
Field: "{FutureLeaveBalance}",
Data: Label1_9.Text
}
)
);
//substitute template data values for placeholder