Don’t worry. You need to give your employer’s name and address in your ET1, and (if different) the address of the place you worked. If your employer is a company, you can find their registered office from the Companies House website. The employment tribunal will send the form to your employer, and tell them their deadline to respond.
If your employer ignores your ET1, that’s good news from your point of view: it means – unless they come up with a very good excuse – they won’t be allowed to take part in any hearing. You’ll get to tell your story, and there will be no-one there to contradict you or cross-examine you. It doesn’t necessarily follow that you will win your case – but it certainly makes life easier. You may even get a judgment in your favour without having to go to a hearing at all.
When you try to enforce your judgment, you may find that your employer suddenly wakes up and starts writing letters saying they never knew about your claim: the ET1 never arrived, or was sent to the wrong address or something. So it is quite important to get your employer’s address right in the first place. If you’re not completely sure about that, you could always send a copy of your ET1 to the place where you worked – possibly addressed to your former line manager – with a covering letter saying something like “This should have arrived at [the company’s registered office, or head office, or whatever you have given as your employer’s address] but just in case there is any difficulty I am sending a copy to you. Please make sure the appropriate people are aware of it.” That will make it very difficult for them to say later that they didn’t know anything about your claim.