I am a postdoc who wishes to apply to a university in the United States for a faculty position in the future. I asked them if I could speak in their seminar, and they invited me.
How should I tell them that I wish to apply there for a faculty position in a year or two, and that they should assess my talk with that in mind?
In universities I have worked at, this is not how junior appointments work. It is more how senior appointments work. You might be able to inquire about the the department and if they expect to have any new lines or retirements in the next couple of years. But not every department gets to hire every year, and often it can be 4-8 years between hires in smaller universities or departments.
They probably already know that you will be on the market in the next couple of years, as you are a postdoc. But it would not hurt to tell them that part. I would not do it in advance, but just as a point of conversation during informal meetings.
How should I tell them that I wish to apply there for a faculty position in a year or two, and that they should assess my talk with that in mind?
You shouldn't. Don't mention it at all. This is like telling a girl on the first date that you plan to propose to her a few months in the future. I assume you have an intuitive grasp of why that might not be the best idea.
So what should you do instead? Actually, you are doing exactly the right thing by getting yourself an invitation to give a seminar. The next thing to do is to go and give it, and give an amazing talk that leaves a really good impression. Also meet with people at the department on the day of your visit and impress them with how smart you are, how interesting the topics you're working on are, how cool your future research plans are, etc.
If you do all of those things successfully, you are setting the ground in the best possible way for a hypothetical day when you apply for a job at that department. And you will be doing all that without saying anything about your wish to someday get a job there -- an awkward topic that can only detract from people's enjoyment of your visit, and is best avoided for the time being.
In the informal talks leading to the seminar, tell them that you would like to apply for a faculty position in the near future, in a year or two.
It will be of mutual benefit if you already address the funding parts, so you can propose to collaborate in bringing in some grants, rather than going there full of requests but with no concrete plan of action.
The seminar after all will be an excellent chance to show all the interested people the research you have been involved, however having slides explicitly "nodding" at the department and future work to be done there by you would be more of harm than anything else, I would perceive it as patronizing.
However, gentle and subtle hints at possible collaboration, to be then discussed informally/orally and in follow up communications would be helpful.
Again, you are proposing some research activity, you should also be able to propose the funding mechanism or the possible industrial partners, and you are expected to do the work on your resources, do not expect much support: everyone wants to jump on a moving train, no one wants to build the railway. If your future faculty position is as advanced as a train almost ready to start, you will have a better reception for your planned goals ...
