How to send notification mail to a specific user's email from the server side through the API?

Thank you so much for the answer.

I followed this and was able to send a notification email but I faced two issues.
1. The custom properties I added while creating the event are not reflected in the email or I am not finding a right way to add the properties in email template. 
 

 const createEventInput = {
      data: {
        type: 'event',
        attributes:   {
          // the below properties are not showing up in the email.
          properties: {
            notification_title: 'This is notification title',
            notification_content: 'This is notification content.',
            content: 'hi, this is content'
          },
          metric: {
            data: {
              type: 'metric',
              attributes: {
                name: 'Web notification'
              }
            }
          },
          profile: {
            data: {
              type: 'profile',
              attributes: {
                email: 'sample@example.com'
              }
            }
          }
        }
      }
    }

2. The email is being sent only once per profile, but my use case is to trigger the flow as many times the user triggers the event ! 
    

Please help me with the answers. Thank you.