Hi,

You can refer to the link specified in the other answer. To be a little more specific with the many2many field, you can try this code.

class XYZConfigSettings(models.TransientModel):
    _name = 'xyz.config.settings'
    _inherit = 'res.config.settings'

    product_ids = fields.Many2many('product.product', string="Products")

    @api.model
    def get_default_values(self, fields):
        IrValues = self.env['ir.values'].sudo()
        product_ids = IrValues.get_default('xyz.config.settings', 'product_ids')
        lines = False
        if product_ids:
            lines = [(6, 0, product_ids)]
        return {
            'product_ids': lines,
        }

    @api.multi
    def set_default_values(self):
        IrValues = self.env['ir.values'].sudo()
        IrValues.set_default('xyz.config.settings', 'product_ids', self.product_ids.ids)

See this Video For Odoo12: How To Save Many2many Field Value In Settings Odoo

Thanks