Laravel Mutator to add predefined values into database

I'm new into Laravel and I'm trying to store the user's company id on a column of the products table each time a user creates a new product. The company's id it's retrieved from the user's session. I'm trying it with Laravel's Mutator:

public function setFirstNameAttribute($value) {
        $this->attributes['company_id'] = session()->get('company.id');
    }

But each time I create a new Product the company id stored it's null. Seems like the function it's never executing. Is there any other resource to perform actions like this?

You must use model events - this will be executed on model creation before saving. Or you can use another events depends on you logic - see docs.

class YourModel extends Model
{
    /**
     * The "booted" method of the model.
     *
     * @return void
     */
    protected static function booted()
    {
        static::creating(function (YourModel $model) {
            $model->company_id = session()->get('company.id');
        });
    }
}

Mutators only works when you change mutating field directly: $model->first_name = 'new_name'

And with your code - you will lost "new_name".