If you have been submitting articles, you will have right away seen the option for guest-writing when you click on 'submit your article'. The feature of Guests submitting articles was introduced a couple of years ago and a policy was framed for them for submitting guest posts. Read the instruction given therein, as well as the links given on that page. Also, if guests are promoting/marketing a business, they have to select that option when submitting the article, and follow the guidelines for promotional content.
Generally- Only one external link is permitted that is relevant to the article and, similarly, a backlink should ideally be provided to the ISC article at that external site. No over-the-top promotion/spamming will be permitted, that is, extolling the external site and giving multiple links to it. All content should also, as you know, follow the articles submission guidelines.
In case you are unsure about the article getting accepted, you can submit your idea through the contact us button that has been provided at the bottom of that page.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell