If all you want to do is store the rep, and the rep is already in your system (ideally as an employee), then you can just add a field to store that information.
The instructions that follow are upgrade safe, meaning this change will persist during database upgrades (including to the next version of Odoo)
1. Activate Developer Mode
2. Navigate to Settings --> Technical --> Database Structure --> Models
3. Search for and open the Sales Order model
4. Click EDIT, scroll to the bottom, click Add an Item
5. Give the new field a name, something like x_outside_sales_rep
6. Give it a Field Label, something like Outside Sales Rep
7. The Field Type would be many2one
8. The Object Relation would be hr.employee (if reps are stored as Employees) or res.partner (if reps are stored as Contacts)
9. Click Save and Close, then Save
This adds the field to the Database.
10. Navigate to Settings --> Technical --> User Interface --> Views
11. Search for and open the sale.order.form view
12. From the Action menu, select Duplicate
13. Change the View Name to sale.order.form.custom
14. Click Save then Edit
15. Change the View Inheritance mode to Extension View
16. Change Inherited View to sale.order.form
17. Delete the contents of Architecture and replace with:
<field name="user_id" position="after">
18. Click Save
This adds the field to the User Interface.
EDIT: I realized you were asking to store this information on the Customer record - in step 11 just use res.partner.form (base.view_partner_form) instead.