the user... mistakes an ideal that we all want/agree on ... as a universal fact in their argument.
It's not an informal fallacy, but a contested premise the claimant is trying to present as uncontested. A series of contested premises is the framing of the argument.
You have your terminology in a bit of a jumble, so we need to lay out a few facts first. An informal fallacy is an argument that is specious, and it is therefore a weak or flawed argument. But fallacies are weaknesses of inference. They involve a bad leap of reason from premises (at least one explicit) to conclusion. Informal fallacies are instances of invalid and weak conclusions drawn in informal logic. However, one can also object to the propositional content of the premises themselves of valid or strong arguments. In this case, there isn't a fallacy, but a bad presumption. Let's show a simplified example:
P1. Astrologers can read the future in the stars.
P2. Mary is an astrologer.
C. Therefore, Mary can read the future.
There is no fallacy here because there is nothing wrong with the inference drawn from this argument! Most philosophers who accept a naturalized epistemology, however, would reject the first premise as unscientific. A scientific epistemology rejects seeing the future with certainty since astrology is a pseudoscience. In other words, there is no logic fallacy in this argument, though the argument would be rejected by any NAS based on a false premise.
Interestingly in law, under certain rules of evidence, certain presumptions cannot be rebutted. In rhetoric, of course, what is and isn't accepted in argumentation is more often what is persuasive than established by a canonical rule set governing debate or argumentation. In informal argumentation, which might be conducted for building consensus or in an attempt to persuade, challenging presumptions usually can be challenged, but often, folkways and mores, the manager's emotional biases or cognitive biases, or groupthink set implicit limits to what can be challenged. If you work for UNICEF, for instance, the mission's charter will essentially select for people who believe certain premises about the rights and welfare of children. If you challenge the premise your manager makes, and they argue that there premise is right by virtue of their authority, then THAT would be a fallacy, specifically an appeal to authority.
When an agent is aware that they are affecting the nature of the argument by selecting certain premises or using loaded language, this is known as framing, which is an old metaphor for deciding what is "in the picture" and what is not. Great debaters win arguments by framing an issue to make it tough to disagree with the premise. For instance, in medical abortion, those in favor of legalizing the procedure frame their arguments as freedom, a right of a woman to choose, whereas those who are against frame it as an act of murder, a crime. If in a debate one does not attack the framing itself, one tends to lose the argument because the nature of language is that the metaphors and connotations tend to operate at a level below conscious awareness in the untrained mind. Let's look at your interest.
In legal drama on television, one often hears the objection in the trial: assumes facts not in evidence! That's exactly what is occurring here. Sometimes, the argument is even more insidious because instead of explicitly stating a false premise, it is insinuated. (See my response to this Philosophy SE question for an example of insinuation.) A debater therefore has a choice. Do they or do they not contest the premise, and if so how, without alienating those you are attempting to persuade. Often, a simple redirect by calling out the presumed premise with a quizzical look is enough to move the argument in the right direction.