How Register and File for Self Assessment from Overseas

I am moving abroad in mid January and will be employed by an employer. I understand that I need to register for self assessment and file a tax return on the income I earn abroad until the end of the tax year, as I have been a resident in the UK this tax year. I have just tried to register and the form asks for the date I began receiving overseas income - obviously I have not started my job yet and don't know the pay date to fill it in in advance. However the form also asks for a UK address, so I won't be able to register once I've left the UK. Can I use my parents' address or does it need to be where I actually live as they will send letters/forms? How/when do I register and file from abroad? I tried speaking to an online agent and he told me I would need third party software, gave me a link to something unrelated, then told me he couldn't help as my question wasn't related to self assessment and ended the chat, so I'm none the wiser.

Hi Emily,
If you want correspondence to go to your parents address then you can update your record for this.
If you have a personal tax account you can do this online here:
Personal tax account: sign in or set up
If you want you can register before you leave the UK and put in an estiamted date if the exact date not known. 
Thank you.