To create a new vacancy on Myn simply navigate to the vacancies tab on the left side bar. From here you can click on the new vacancy button in the top right. From here you will be asked to choose which project you want this vacancy to be created in and also the title for this vacancy.
From here there are 4 steps in creating a vacancy on the Myn platform. Below is a help doc on each page, and what information is required on each of those pages to get the best results for your vacancy.
Creating a new Vacancy - Description
Creating a new Vacancy - Details