As of r2020a, the typical ToolBar is not (yet?) supported for UIFigures (eg, apps) but the property exists which usually means that support is coming soon.
This is how you would turn on the toolbar for UIFigures from your StartupFcn, if/when it becomes supported.
app.UIFigure.ToolBar = 'Figure'; % or 'auto'
% This will result in an error in the current release, r2020a.
The first input to your startupFcn is the app variable which can be passed to your toobar callback functions.
% Example
function startupFcn(app)
tb = uitoolbar(app.UIFigure);
pt = uipushtool(tb);
pt.ClickedCallback = {@myfunc,app};
end
Then set up the callback function. In r2020a, an error occurs when the callback function is defined within app designer code, even if the callback function is public. So, the callback function must be defined in an external m-file until this problem is fixed. Update: Alternatively, seeSreerup Banerjee's comment below describing how the callback function can be hosted by the app.
function myfunc(scr, event, app)
% do stuff
end
