The number of solutions to $a+b=1$ in a multiplicative subgroup of a finite field $GF(q)$

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I have been wondering what is the number of solutions to $a+b=1$ in an arbitrary multiplicative subgroup $H$ (order $r$) of some finite field $GF(q)$, where $q=p^n$ is a prime power and $a, b \in H$. If there is no known results for a general $q$, what about the case $q$ is power of $2$? I'm trying to find a general upper bound on the number of solutions, depending on $p,n$ and $r$.