Hi,

Inherit the corresponding model and add the compute to corresponding field.

class ModelName(models.Model):
    _inherit = "model.name"

    so_line = fields.Char(compute='_compute_function_name')

To do it from user interface, use automated actions in odoo. See a use case here: https://www.youtube.com/watch?v=Cz5eM5FDmTE&list=PLqRRLx0cl0hq6DeOzGlfHyjgBxjN5jW90&index=1

Thanks