Before any session, be it face-to-face or at a distance, it is important to have prior permission from the recipient.
It doesn't hurt to apply Reiki without the other person knowing, for example: the husband fell asleep and knowing that it will do him good, a session is given there, or our mother is not doing well and we send Reiki from a distance... it won't hurt either the person or the therapist but there are some necessary reflections.
Generally, if we want (we are not supposed to) do Reiki without a person knowing it, it is because we know that he does not want to receive it, or simply does not believe. By insisting on this we are manifesting our own will, a desire that is ours, even if we think it is for the good of that person. Example, we have a stressed husband and we want him to become a calmer person, but he doesn't want to receive Reiki or doesn't believe. Would it help with sessions without knowledge? maybe... but if he didn't make changes in his own life, Reiki wouldn't bring about those changes. And the truth is that we would be interfering with the free will of the person concerned, which is not correct.
We then have to reflect on our desires when we want to apply Reiki to someone who does not want to receive it. And maybe, working on acceptance in the face of other people's choices, maybe these people have to go through certain trials in their evolutionary process in this life, no matter how much it costs us. We have no right to force anyone to do what they don't want to do.
There are some exceptions, such as babies or children, or people who for some reason are not in a position to make a choice, people who are in a coma for example, in these cases we can ask for authorization from whoever is responsible for that person, or in the case of whether we are ourselves or not, we ask the person's "Higher Self" for permission, there are ways of feeling whether we can or cannot do it.
There are also other ways to not hurt free will and help. We may not do Reiki on the person himself, but we can do Reiki on the space he frequents, on the bed where he sleeps, on the clothes he wears, on the food he eats, on the water he drinks, on the medication he takes. And all this will end up influencing the person in a positive way, without us having applied Reiki directly to them, thus respecting their will / belief, or lack thereof.