Hi @Shri Shankran ,
Per my understanding, "http://services.myportal.com/employee/advisees" should be an external domain custom web service which is outside SharePoint Online.
Then it's necessary to add Access-Control-Allow-Origin header in your custom web service, for example here is a demo to add custom header in .NET Web API Web.Conig:
Access-Control-Allow-Origin Header and the ASP.NET Web API
Here is a similiar question for your reference:
AJAX POST REST call generates CORS error
If the response is helpful, please click "Accept Answer" and upvote it.