Report this to the police
"do not worry it is not a big deal, just sign this paper and I will
write the warning paragraph later".
Your boss has literally stated outright that she intends to forge a document. This is a serious crime in all jurisdictions and nations that I am aware of. Moreover, the implications could be anything from severe to devastating to you personally (and the company too).
I like the other answers, but I would most certainly report this to the police as well. Your boss has admitted to planning something criminal.
The police will almost certainly have to write a report on what you tell them. Although I do recommend documenting this with HR as well, HR may or may not do their job, and the police is much more reliable to document this properly for you. If worst comes to worst (which, indeed, it very well may), and your job ends up on the line, having this documented by the police can be very useful. You may end up in court, and having this documented by the police may be a big help.
In the best case scenario, the police gets a search warrant, goes to the office of the boss, and finds the blank piece of paper, backing up your story. (I realise this may be an unlikely outcome, but your story is the type which triggers the "justice" part in the conscience of people, and therefore it is not impossible.
I realise that reporting your boss to the police is a rather aggressive approach, but this is also a quite extreme situation. Your boss has already admitted an intent to commit fraud against you, and the company. She has already taken the gloves off. You have very little to lose here. Your boss deserves to lose her job over this, not you.
Go to your/a union
Also, if you are not a member of a union, join one now, and ask them for assistance. They typically have legal teams which can help in such situations.
Consult a lawyer
If somehow a union can not provide a lawyer for you, consider consulting one on your own, if you can afford it.
Try to get the paper back, or to get evidence of what happened
This last part has only a mediocre chance of success, but you might as well try. There is a downvoted (possibly for typesetting, not content) answer which suggests talking to the boss about it, while secretly recording it, hoping that she somehow admits to what she has done, given that you are located in one of the few jurisdictions in which that is legal. If you are sure that you happen to find yourself in one of those jurisdictions, then I find that idea reasonable. If she does admit that and you get it recorded, that would be a rather valuable defense to you, and it would no longer be her words against yours. If you do happen to have such a conversation, you can, in the same conversation, ask for the paper back. This will put her on the spot, and it will be difficult for her to hold on to the paper without admitting that it exists. Also, there is the chance that she may give it back voluntarily. In that conversation, you may actually be polite. You can tell her that you thought about it, and you would like to see the warning before you sign it, and therefore want the blank paper back. If she refuses, you can step it up a notch, and inform her that what she is doing is illegal. After all that, I would say you have a pretty good chance of either getting the paper back, or getting a recording of her admitting what happened. In the worst case scenario that she just pretends she has no idea what you are talking about, and does not give you anything, then you have not really lost much, and you can proceed with HR, the police, a/your union, and a lawyer.
If recording without knowledge is not legal in your jurisdiction, I do not see why you should not have the conversation anyway, without recording. After all, what she is doing is pretty insane, and there may be a chance she has an ounce of sense in her and decides to give it back, fearing the possible consequences of committing fraud.
Act NOW, do not wait
Time may be of the essence here. Having this documented and dated before the manager shows her forgery to anyone else and has that recorded, may be quite important for getting people to believe you here. Also, by acting quickly you can make something happen before she even writes anything on the sheet in the first place. The sheet of paper with nothing but your signature on it is the single most damning evidence here if found in the possession of your boss. Note that regardless of what the manager writes on that paper, it will be a forgery, and criminal. I would not waste even a second here, I would immediately get this documented.