You can set a password on the Settings tab for any of your Treejack, OptimalSort, Chalkmark and Questions studies.
If you enter a password into the text box, only participants with that password will be able to complete your study. If you leave it blank, then anyone with the link can complete your study.
If you are only sending your study link to people you know, or if you don't mind who participates in your study, you probably don't need to use a password. You can't change your password setting after you launch your study.