In order to promote a specific service or freelancer on Legiit you will have to have made a purchase from that freelancer in the past.
If you have, and they are not a premium seller that has affiliate turned off for that service(s) an affiliate link will appear and you can use it to promote that service or freelancer.
If you have a freelancer whom you have purchased form and would like to promote, and they have affiliate turned off, you can message them and ask them to enable affiliate for that service, or their entire profile.
If they do, the affiliate link for that service will become available and work as described above.
You can also promote Legiit as a whole by going to Menu > Settings > Affiliate Settings and copying your link.
When someone clicks your affiliate link they will be cookied to you for 7 days (or until another affiliate over rides your cookie) and you will get 15% commission on anything they purchase, except services that have affiliate turned off.