The graph was made in Matlab with the following source code

% Function data
[X,Y] = meshgrid(-1.2:0.01:1.2, -1.2:0.01:1.2);
Z = X+Y;

% Constraint data
t = -pi:0.01:pi;
xc = cos(t);
yc = sin(t);
zc = xc+yc;

% Plot graph and 'constraint'
figure();
meshc(X,Y,Z);
hold on;
plot3(xc, yc, zc, 'k', 'LineWidth', 2);
plot3(xc, yc, -3*ones(1,length(t)), 'k', 'LineWidth', 1);
set(1, 'Color', [1 1 1]);
set(1, 'Position', [46 107 767 682]);
% Axis labels
text(0.2, -2.2, -3, 'x');
text(2.0, -.3, -3, 'y');
text(-1.8, -1.5, 0, 'z');

% Annotations of the max and min points
% Point indicating max
plot3(sqrt(2)/2, sqrt(2)/2, 1.45, 'kx', 'MarkerSize', 14, 'LineWidth', 2);
plot3(sqrt(2)/2, sqrt(2)/2, -3, 'kx', 'MarkerSize', 10, 'LineWidth', 2);
text('Interpreter', 'latex', 'String', '$$(\sqrt{2}/2, \sqrt{2}/2, \sqrt{2})$$',...
	'Position', [.0 .3 1.9], 'FontSize', 20);
% Point indicating min
plot3(-sqrt(2)/2, -sqrt(2)/2, -sqrt(2), 'kx', 'MarkerSize', 14, 'LineWidth', 2);
plot3(-sqrt(2)/2, -sqrt(2)/2, -3, 'kx', 'MarkerSize', 10, 'LineWidth', 2);
text('Interpreter', 'latex', 'String', '$$(-\sqrt{2}/2, -\sqrt{2}/2, -\sqrt{2})$$',...
	'Position', [-1.4 -1.0 -1.0], 'FontSize', 20);
hold off;
view([34 24]);