Generate Sequence Numbesr in Odoo | What is Sequence Number in Odoo

create_sequence_number_in_odoo.py

class your_model(models.Model):
    _name = 'your.model'

    application_no = fields.Char('Application No.', default='/')

    # on create method
    @api.model
    def create(self, vals):
        obj = super(your_model, self).create(vals)
        if obj.application_no == '/':
            number = self.env['ir.sequence'].get('your.sequence.code') or '/'
            obj.write({'application_no': number})
        return obj
        
    
    # on button click event
    @api.one
    def submit_application(self):
        if self.application_no == '/':
            sequence_id = self.env['ir.sequence'].search([('code', '=', 'your.sequence.code')])
            sequence_pool = self.env['ir.sequence']
            application_no = sequence_pool.sudo().get_id(sequence_id.id)
            self.write({'application_no': application_no})

Read more in detail: https://learnopenerp.blogspot.com/2020/08/generate-create-sequence-number-odoo.html