How to set 'unrated' in live chess?

I would like to play longer (30 min/45 min) games unrated, but for the love of all holy, I can't find the option anywhere in the settings (website), only the rating settings.
Please help. 

It's hard enough to find good opponents for slow rated standard games online. You want someone to spend hours with you yet you are not willing to provide any reassurance to them that you will take the game seriously. 

But if you insist, just choose "Custom" from the list of time choices. Put in the time you desire. Then select "Unrated". Although the time request disappears, it does make it into the seek.  So if you want to seek a  97 17 unrated game, it's all up to you. 

ANYONE, even first time players, can and will accept your challenge. The hard part may be finding a serious player worth sitting at your computer for 15 minutes while you try to decide if he is still pondering or has gone on to other things.  Achieve a winning position and many of them will just disappear.

What you really want is an unrated game with a rated player. That is something I can't tell you how to achieve, other than playing games against "Friends".

Therefore, you are left with playing serious rated games against rated players. The ratings are virtually meaningless here, but people do take them somewhat seriously and try to protect them. Your unwillingness to risk a few measly rating points makes it less likely you will have a good game.

The reason is nothing like that.
It's simply that I'm at work and I have not the time, nor the focus to play short rated games, so I just would like to leave a game on, switch to another tab when the boss is around, switch back when he's gone (yes, I do have an easy job with lots of free time).

Thanks, though.
Also thanks for the utterly condescending style and prejudging. 

You really need to play against computers, not real people. Real people will get upset when you disappear in the middle of the game without any explanation and it is not reasonable to ask complete strangers to have the patience to sit around for 25 minutes waiting for you to come back.  

Perhaps you think that the game is unrated so it doesn't matter.  Yes that is true if you have no regard for other people.

Most of us, even on the internet, believe that certain minimum standards of behavior are expected.  Your plan is completely unacceptable for playing against humans. 

Computers won't get upset. They won't even call time on you in case you have to work. 

In fact, I've already played 2 of those games in the next hour.
I really don't think it has anything to do with a 'minimal standard of behaviour' and being 'unaccapable' as you put it, as I'm clear as day setting the circumstances of the game, and once they accept it, it's between two people, not me forcing my way of playing on the other.
I also inform them about the situation I'm in, and offer a draw if they disagree for waiting about 3-4 minutes for one move (but I guess they don't, since they accept a 30 minutes match in the first place).
Take a chill pill. You're overreacting.