Extra Field in write blog

I need to create a textbox for a “links” field below the content edit box so i can also include these links (hidden) to each post

basically you will view the post and below will be a download link, this link will redirect you to another page with a 10 second countdown and after this it will show the links (ajax from another page which pulls them from the db)

i am having trouble adding this field to wordpress and making sure it actually saves and publishes correctly with the posts.