To set a project's start date, use the drop-down menu located next to the project name, select "Edit project details", open the Due Date calendar and add a START DATE.
There's one thing they don't explain, though, that can be a little confusing: when entering a date, Asana does not let you enter just the "start date" portion and leave the "due date" portion blank. If you do that, Asana transfers the "start date" you entered to be the "due date" and blanks out the "start date" - and worse, it doesn't warn you that it did that!
Furthermore, the dates you enter for start and due date must be two different dates - if you enter the same date for both start and due, Asana does the same thing as described above - it uses that date as the due date and blanks out the start date!
What does this mean for Flowsana? Well, when entering a project start date for Dynamic Duration workflow purposes, Flowsana doesn't use or care about the project due date. But the above-described Asana behavior means that you must enter a project due date anyway. Again, it doesn't matter to Flowsana what you enter as the due date - all it looks at is the start date - but you have to enter some due date just to get the start date to be recorded.