To enable the supplier contacts of a parent company to respond to the negotiations of their subsidiaries, the parent supplier must be added to the negotiation as an invited supplier. This will allow the parent supplier contacts to access the negotiation through the Oracle Supplier Portal and submit responses on behalf of the subsidiary supplier. The parent supplier can also view and monitor the negotiation progress and status. The parent supplier must have an active supplier portal user account and the appropriate roles and privileges to participate in the negotiation.
References:
How You Invite Suppliers to Negotiations1
Oracle Supplier Negotiations2
Response to Negotiations3

See below in Explanation for each Step.
Explanation:
To define a Procurement Agent, you can follow these steps:
* Navigate to the Setup and Maintenance work area and search for the Manage Procurement Agents task.
* Click on the Go to Task icon to open the Manage Procurement Agents page.
* Click on the Create icon to create a new procurement agent.
* Enter the following information in the Create Procurement Agent dialog box:
* Procurement Business Unit: US1 Business Unit
* Agent: Student, PRC01
* Procurement Actions: Select all the check boxes to assign all the default procurement actions
* Click on the Save and Close button to save the procurement agent.
You have successfully defined a procurement agent. You can verify the procurement agent details by searching for it in the Manage Procurement Agents page. For more information on how to define and manage procurement agents, you can refer to the Define Procurement Agents (Oracle Fusion Cloud Procurement Implementation Guide) document.
Or the following steps:
Following the scenario, we need to define you (Student, PRCXX) as a Procurement Agent with access to manage procurement transactions in the US1 Business Unit.
Here are the steps to define the Procurement Agent:
* Navigate to Manage Procurement Agents:
* Go to the Global Navigation Menu.
* Click on Procurement.
* Click on Setup and Maintenance.
* Click on Procurement Agents.
* Create the Procurement Agent:
* Click on the Create icon (+ icon).
* Enter the Procurement Agent Information:
* Procurement BU: Select "US1 Business Unit".
* Agent: Enter your assigned Login username - "Student, PRCXX" (replace xx with 01).
* Requisitioning BU: (Optional) You can select a specific Requisitioning Business Unit if you primarily handle requests from that unit. Otherwise, leave it blank.
* Default Procurement Actions: Select "Use Defaults". This assigns all default procurement actions (create purchase orders, issue requisitions, etc.) to you.
* Save the Procurement Agent:
* Click on the Save button.
Verification:
* You should now be listed as a Procurement Agent in the Manage Procurement Agents page.
* You can verify your assigned actions and Business Units by clicking on your record.
* You should be able to create and manage procurement transactions within the US1 Business Unit.
Additional Notes:
* Make sure to replace "xx" with your allocated User ID (01) in the Agent field for accurate identification.
* Selecting "Use Defaults" assigns all standard procurement actions to you. You can customize these actions later if needed.
* Remember that your access level and permissions within the Procurement module will be determined by your Procurement Agent role and associated Business Units.

To send your purchasing documents from a specific organization-wide email address, you need to use the task Manage Purchasing Profile Options. This task allows you to configure various profile options that control the behavior and functionality of the Purchasing application. One of these profile options is PO: Email Sender Address, which specifies the email address that appears in the From field of the email notifications sent to suppliers and internal users. You can set this profile option at the site level to apply it to all business units, or at the product level to apply it to a specific business unit.
:
Implement Procurement - docs.oracle.com
PRC:PO: Supplier Communication from Email Address ... - My Oracle Support PRC:PO: Sending Purchase Order (PO) Email ... - My Oracle Support

To simplify supplier self-service registration and bypass mandatory supplier identifiers for some regions, you should use the Configure Supplier Registration and Profile Change Request task in the Setup and Maintenance work area. This task allows you to define the registration settings for prospective and spend authorized suppliers, such as the required attributes, the approval rules, and the site assignments. You can also specify the supplier identifiers that are mandatory or optional for each country or region. For example, you can make the tax registration number optional for suppliers in a region where it is not commonly used.
:
Options to Configure Supplier Registration and Supplier Profile Change Request, Section 1: "Options to Configure Supplier Registration and Supplier Profile Change Request" Configure Supplier Registration and Profile Change Request, Section 1: "Configure Supplier Registration and Profile Change Request"