To add your application link or ATS URL to your posting, go to the Contact Information section on the Post a Job form. This is the second to last section in this form before you complete your posting. Here is what that looks like:
Please note that you should not add any contact information such as name, email address, or phone number to this section if you only can accept application via your provided URL. This is also true for the Applications section; make sure that the box next to Allow Online Applications is not check or selected (as shown above).
When a job seeker clicks the Apply Now button on the job search page, they will be redirected to the link you have provided.