See explanation below.
Explanation
You need to configure a Sensitivity label.
* Go to the Security & Compliance Admin Center.
* Navigate to Classification > Sensitivity labels.
* Click on + Create a label to create a new label.
* Give the label a name and description then click Next.
* Leave the Encryption option as None and click Next.
* On the Content Marking page, tick the checkbox
* Click the Customize Text link and add the footer text and click (for the question, it doesn't matter what text you add).
* Click Next.
* Leave the Auto-labeling for Office apps
* Click the Submit button to save your changes.
* The label is now ready to be published. Click the button to exit the page and create the label.

Explanation

Reference:
https://docs.microsoft.com/en-us/cloud-app-security/data-protection-policies

See explanation below.
Explanation
1. Navigate to eDiscovery in the Security & Compliance Center, and then click Create a case.
2. On the New Case page, give the case a name, type an optional description, and then click Save. The case name must be unique in your organization.

The new case is displayed in the list of cases on the page. You can hover the cursor over a case name to display information about the case, including the status of the case (Active or Closed), the description of the case (that was created in the previous step), and when the case was changed last and who changed it.
To create a hold for an eDiscovery case:
1. In the Security & Compliance Center, click to display the list of cases in your organization.
2. Click Open next to the case that you want to create the holds in.
3. On the Home page for the case, click the Hold tab.

4. On the Hold page, click Create.
5. On the Name your hold page, give the hold a name. The name of the hold must be unique in your organization.

6. (Optional) In the Description box, add a description of the hold.
7. Click Next.
8. Choose the content locations that you want to place on hold. You can place mailboxes, sites, and public folders on hold.

* Exchange email - Click Choose users, groups, or teams and then click Choose users, groups, or
* teams again. to specify mailboxes to place on hold. Use the search box to find user mailboxes and distribution groups (to place a hold on the mailboxes of group members) to place on hold. You can also place a hold on the associated mailbox for a Microsoft Team, a Yammer Group, or an Office 365 Group.
Select the user, group, team check box, click Choose, and then click Done.
Note
When you click Choose users, groups, or teams to specify mailboxes to place on hold, the mailbox picker that's displayed is empty. This is by design to enhance performance. To add people to this list, type a name (a minimum of 3 characters) in the search box.
9. After configuring a query-based hold, click
10. Review your settings, and then click
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/ediscovery-cases?view=o365-worldwide#step-4-pla