To apply a job, The applicant need to log in to his/her account. Then go to the job details page which he/she want to apply.
If the applicant is applying to a job for first time, He/She need to update his/her profile. Once the profile is updated,
Then he/she can see a cover letter field at the bottom of the page. Other information will be picked from the applicant's profile.
If there is any external link, Then the apply button will redirect the applicant to the company's job application page.