Ontologically, there may or may not exist a substance 'self' (be it the ātman or the brain). But when does one become aware of oneself as a subject? Prābhākara authors maintain that this only happens when an injunction (e.g., "the one who is desirous of heaven should sacrifice") addresses one, so that s/he can recognise herself/himself as the enjoined one and recognise in herself/himself what is said about him/her in the injunction.
But how can an injunction be uttered if it does not yet address any subject? A Vedic injunction does not concern a specific person, since it lacks any temporal dimension and hence any possible link with specific individuals. Rather, the injunction constitutes one as a subject insofar as it makes one aware of him- or herself as being so and so (in the Prābhākara account: as being desirous).
Prābhākara thinkers are mainly concerned with Vedic injunctions, but one could add that, in a similar way, a child acquires gradually awareness of him- or herself as a person, through the injunctions of his/her parents. Injunctions to children start to be uttered well-before they can be conceived of as subjects (and, consequently, as able to understand the order enjoined to them). By hearing them, the child him- or herself as the enjoined one, and, hence as one which can be enjoined, a person.
A young child starts becoming aware of things around him or her and, hence, implicitly, of the fact that s/he is experiencing them. It is only through other people's injunctions, however, that s/he becomes aware of that experiencer as being himself/herself. This awareness of himself/herself, in summary, embeds the cognitive capacity within an intersubjectively shared personhood.
On this topic, see also:
Does a phenomenological approach lead us to a self-as-consciousness?
Is the ātman 'me'?
(and many other posts with label "subject")