But no one is going to get male T levels just with the power of belief. Placebo gets you so far within the limits of your body. If your body has the means to fight infection and you believe that it can, it boosts the immune system. But with T you can believe all you like but the body just doesn't have the means to produce enough T. If there was a study of FTM pre-T VS CIS females, I'd bet that T levels were found, on average, to be higher for FTM. Most find they have fairly high levels already when going for the blood tests before starting.
The placebo effect is in the mind boosting what the body is capable of. If the body has the means, a placebo will boost the effect. But, if the body doesn't have the means, it's not going to happen. A person who needs a heart transplant can believe with all their heart (no pun intended) that they'll survive, and that belief can indeed help them survive longer, but it can't fix their heart, without an actual new heart they're not going to survive. The exact same applies to T.
The evidence for that is all around us here. It's why FTMs need T if they want to go that far. For many of us we've felt like we should have been boys from the get go, at that point the placebo effect is 100% engaged because we don't know any better. We can want our bodies to be male 100%, and yet that belief and denial that we're female won't stop puberty. Frankly the placebo effect has already happened to the highest degree possible, now there is undeniable doubt so it's not as strong an effect now.