how to plot a red dot?

For some reason I get a mirrored version of your output, but you can just add whatever you like using TikZ methods.

documentclass{standalone}
usepackage[compat=1.1.0]{tikz-feynman}
begin{document}
centering
begin{tikzpicture}
begin{feynman}
vertex (a) {(b)};
vertex [below= 2.4cm of a] (b){(g)};
vertex [below right=of a] (c);
vertex [ right=of c] (d) ;
vertex [below right=1.0cm of d] (f3);
vertex [below right=1.0cm of f3] (f4);
vertex [above right=1.3cm of f3] (f5) {(h)};
vertex [above right=1.3cm of d] (fw){(W^{-})};

diagram* {
(a) -- [fermion] (c) -- [fermion] (d),
(b) -- [gluon] (c),
(d) -- [fermion,edge label'=(t)] (f3) --[fermion,edge label'=(t)] (f4),
(f3)-- [scalar] (f5),
(d)--[boson] (fw),
};
end{feynman}
path (f3) node[red,circle,fill,inner sep=1.2pt,label={[red]below left:{$y_t$}}]{};
end{tikzpicture}
end{document}