I was able to figure it out. I added a part for a line(plot3) and for a marker (scatter3).
wn1 = 21*rand(100000,1);
zeta1 = 100*rand(100000,1);
wn1zeta1_matrix = [wn1 zeta1];
hold on
hist3(wn1zeta1_matrix,'Edges',{0:5.25:21 0:5:100})
N=hist3(wn1zeta1_matrix,'Edges',{0:5.25:21 0:5:100});
maxN = max(N(:));
colorbar
set(gcf,'renderer','zbuffer');
set(get(gca,'child'),'FaceColor','interp','CDataMode','auto');
scatter3([1.5 1.5], [1.5 1.5],[maxN maxN],25,'w*'); %adds a marker
plot3([0 0.4246*100], [0.4246*100 0],[maxN maxN],'w','LineWidth',1); %adds a line
hold off
axis([0 21 0 100])
% set(gca,'YTick',[-14 -10 -5 0 5 10 15])
xlabel('{\omega}_n (rad/s)')
ylabel('{\zeta} (%) ','rot',0)
title('Histogram of Damping Ratio versus Natural Frequency')
set(gca,'FontSize',10,'fontWeight','bold')
set(findall(gcf,'type','text'),'FontSize',10,'fontWeight','bold')
