How can I formulate an argumentative thesis about Wilfred Owen's poem "Strange Meeting"?

This is a broad question, and the poem gives you a lot of scope for argument. If we consider first what you are being asked to do here, this might give an indication of what the options are.
The phrase "argumentative thesis statement" simply makes explicit what any thesis statement should be—that is, it should present an argument which could be refuted or debated by somebody presenting a strong opposing argument. As such, a thesis statement presents your opinion about something, but then defends this opinion or interpretation with facts. Your statement should also be reasonably focused on one particular aspect or point, because otherwise it would become too broad. So, a bad thesis statement for this poem might be:
"In this poem, Owen presents a negative view of war."
It isn't really possible to debate this, and no scholar would disagree with it. It's also far too broad and does not focus in on a particular aspect.
A better thesis statement should focus in on, for example, the language of Owen's poem:
"In 'Strange Meeting,' Wilfred Owen uses archaic language and evoke religious imagery to convey the unreal horror of war by portraying it as an aspect of hell itself."
You can support a thesis statement like this one by picking out particular language choices (the sleepers "lifting distressful hands" suggests medieval depictions of sinners in hell; words like "sullen hall," "citadels," and "titanic wars" suggest ancient things, and give a timelessness to the situation). The word "Hell" appears, of course. The speaker's language is extremely formal; he is described as a "vision"—this poem could be compared to Dante's meetings with spirits in the circles of hell, or Greek mythology's depiction of spirits speaking to the living in the underworld. The speaker's use of archaic sentence structures ("for so you frowned", "when much blood...") compounds this idea.
Of course, there are many other thesis statements you could make about this poem, but I hope this gives you some idea of where to start and how to support your thesis statement in order to make a strong argument.