I start with some known preliminaries on the problem:


**Classical result.** The one-dimensional Cauchy functional equation
$$
\forall x,y \in \mathbb{R}, \,\,\,f(x+y)=f(x)+f(y)
$$
with $f:\mathbb{R}\to \mathbb{R}$ is only solved by the trivial solutions $f(x)=cx$, for some $c \in \mathbb{R}$, if $f$ satisfies for some additional conditions, e.g., continuity.


**Classical result with restricted domain.** Now let $\mathbb{R}^+:=(0,\infty)$. It is clear from the proof of the above classical result that if $f:\mathbb{R}^+\to \mathbb{R}^+$ is a continuous function such that
$$
\forall x,y \in \mathbb{R}^+, \,\,\,f(x+y)=f(x)+f(y) \, ,
$$
then there exists $c \in \mathbb{R}^+$ such that $f(x)=cx$ for all $x$.


**Multidimensional Cauchy functional equation.** It is also well known that if $f:\mathbb{R}^2\to \mathbb{R}$ is a continuous function such that
$$
\forall x,y \in \mathbb{R}^2, \,\,\,f(x+y)=f(x)+f(y),
$$
then there exist $A,B \in \mathbb{R}$ such that $f(x,y)=Ax+By$ for all $(x,y) \in \mathbb{R}^2$.


I know that the following generalization holds true as well. In particular, I already know how to prove it, by using a variant of the classical proof. In the following, a cone $C\subseteq \mathbb{R}^2$ is a set for which $\alpha x+\beta y \in C$ whenever $\alpha,\beta \in \mathbb{R}^+$ and $x,y \in C$.



> 
> **Fact.** Let $C\subseteq \mathbb{R}^2$ be a non-empty cone and $f:C \to \mathbb{R}$ be a continuous function such that
>  $$
> \forall x,y \in C, \,\,\,f(x+y)=f(x)+f(y).
> $$
>  Then there exist $A,B \in \mathbb{R}$ such that $f(x,y)=Ax+By$ for all $(x,y) \in C$.
> 
> 
> 


Is it a known result? In such case, does anyone have a reference for this result?



On a generalization of the classical Cauchy's functional equation
As I have just learned from Janusz Matkowski, this follows from Theorems 5.5.2 and 18.2.1 in Kuczma's book (the same mentioned in my comments to the OP), after ruling out the trivial case when the cone $C$ is a line or a half-line. In particular, Theorem 5.5.2 is about the (unrestricted) Cauchy functional equation in $\mathbf R^n$, and shows that the only continuous solutions are precisely the functions of the form $\mathbf R^n \to \mathbf R: (x\_1, \ldots, x\_n) \mapsto \sum\_{i=1}^n c\_i x\_i$ with $c\_1, \ldots, c\_n \in \mathbf R$, while Theorem 18.2.1 reads as follows:



> 
> Let $G$ and $H$ be abelian groups (written additively), and let $S$ be a subsemigroup of $G$ such that $G = S - S := \{x-y: x, y \in S\}$. If $g : S \to H$ is a (semigroup) homomorphism, then there exists a unique
>  homomorphism $f : G \to H$ whose restriction to $S$ is $g$.
> 
> 
> 


Each of these results is somehow a piece of folklore (for people
working primarily on functional equations and related topics), but I think it is not harmful to have a reference.


