Discrediting your opposites is primarily a feature of rhetoric, not discourse.
If your goal is to have an earnest discussion, to go over the various sorts of evidence and the varied hypotheses, you need to address those first.
Some American scientists have claimed the virus does not look engineered. Why? What is their evidence for the claim? If you are asserting the virus is or at least could be engineered, can you account for the evidence supporting their assertion? Can you point to evidence that supports your hypothesis and not theirs? How do they respond to this evidence?
It may be -- in an earnest discussion -- that one side is not engaging, for one reason or another. Maybe they don't want to admit they are wrong, maybe they are getting paid not to. At that point, after you have yourself examined their claims, after you have examined whether you are just being stubborn yourself, it is quite reasonable to disregard them, maybe even in the future.
Bringing it in too early, before engaging with their claims, trying to find reasons not to examine their evidence, makes it seem like you are just interested in winning the rhetorical argument.