I'd always suggest that everyone make an Advance Decision about the sort of care they want if they are unable to communicate. However, it's worth being realistic about what this can and can't do.
Legally, you can withhold consent for treatment in advance. So you can say "I don't want CPR". "I don't want ventilation". "I don't want blood products".
You can't consent to treatment in advance. And you can't use it to force doctors to provide care they believe is hopeless or not clinically justified.
So, even if you have written you want ITU care, if it isn't clinically justified you will not be offered it.
An intensivist colleague has a poster which says, amongst other things, "CPR is not a cure for normal dying". It's worth remembering that. Modern medical care can achieve remarkable things, but it isn't a cureall, and sometimes the kindest thing to do is ease suffering and let nature proceed.
It's also worth having a discussion with your loved ones about what your wishes and preferences would be. As @Northsoutheastwest76 says, these differ between individuals. If they end up having to input into decision making it is hugely helpful for everyone - emotionally and practically - if they know what you would want if you were able to communicate.
And make sure, if you have written something, that everyone knows where it is! I have a copy with my will, and other family members have copies, too. It's no use if it's buried in your sock drawer and noone knows to look.