Firstly, you will need to create a one or many different ticket type(s) for the pre-sales, here we've gone for 'Friends and family', who we have chosen to give an opportunity to purchase reduced price tickets. You can have as many as you'd like.
Then, you should set a discount code for the tickets you want to be available for pre-sales, which you can distribute to the people you'd like to be able to purchase those types of tickets. To do this, click 'Options' next to the ticket type(s) you want to distribute, scroll to the bottom of the dropdown section and add in your desired secret code into the box marked 'A password or discount code people will have to enter to purchase this ticket type'
Then, you need to make sure people without the code can't purchase general release tickets early. Click 'Options' next to the ticket types which will not be available for pre-sale.
About halfway down the dropdown section, you can choose to have that ticket type go on sale from either a fixed point from the beginning of a performance (e.g. 2 weeks before a performance starts). This will be different for each performance, so if you have a performance on a Friday evening and a Saturday evening and choose two weeks beforehand - tickets will be available for the Friday two weeks beforehand but tickets for the Saturday performance will go on sale the next day.
Or you can choose to have it go on sale from a particular date (at 00:00 on the morning of that date). This will apply equally to all performances, so if you have a Friday and a Saturday performance and choose for that ticket to go on sale from the 10th October, customers will be able to purchase tickets for the Friday and the Saturday on the 10th
Once you've done that, customers going to purchase tickets will see something like this, if they have the pre-sales discount code they will be able to enter it and make their purchase