See explanation below.
Explanation
1. Navigate to Manage Alerts in the Security & Compliance Center.
2. On the Activity alerts page, click + New.
The flyout page to create an activity alert is displayed.

3. Complete the following fields to create an activity alert:
a. Name - Type a name for the alert. Alert names must be unique within your organization.
b. Description (Optional) - Describe the alert, such as the activities and users being tracked, and the users that email notifications are sent to. Descriptions provide a quick and easy way to describe the purpose of the alert to other admins.
c. Alert type - Make sure the Custom option is selected.
d. Send this alert when - Click Send this alert when and then configure these two fields:
* Activities - Click the drop-down list to display the activities that you can create an alert for. This is the same activities list that's displayed when you search the Office 365 audit log. You can select one or more specific activities or you can click the activity group name to select all activities in the group. For
* a description of these activities, see the "Audited activities" section in Search the audit log. When a user performs any of the activities that you've added to the alert, an email notification is sent.
* Users - Click this box and then select one or more users. If the users in this box perform the activities that you added to the Activities box, an alert will be sent. Leave the Users box blank to send an alert when any user in your organization performs the activities specified by the alert.
e. Send this alert to - Click Send this alert, and then click in the Recipients box and type a name to add a user's who will receive an email notification when a user (specified in the Users box) performs an activity (specified in the Activities box). Note that you are added to the list of recipients by default. You can remove your name from this list.
4. Click Save to create the alert.
The new alert is displayed in the list on the Activity alerts page.

The status of the alert is set to On. Note that the recipients who will receive an email notification when an alert is sent are also listed.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/create-activity-alerts?view=o365-worldwide