Thank you for your reply, Should i add the asset file in the manifest ?

yes,add js file reference according to odoo version

i have already added and edited everything a you mentioned. But nothing changed regarding the datetime picker.

Manifest:
....
'assets': {
'web.assets_frontend': [
'attendance_regularization/static/src/js/custom_script.js',
]
},

custom_script.js :

odoo.define('attendance_regularization.custom_script', function (require) {
"use strict";

var core = require('web.core');
var FieldDateTime = core.form_widget_registry.get('datetime');

FieldDateTime.include({
pickerDateTimeFormat: 'YYYY-MM-DD HH:mm',
});
});

Portal view:

<div class="row">
<div class="form-group col-xl-3">
<label class="col-form-label" for="from_date"><b>From Date:</b></label>
</div>
<div class="form-group col-xl-6">
<input type="datetime-local" name="from_date" class="form-control" required="true" />
</div>
</div>

am i missing something here ?