Show dialog to choose workspace while it starts

You can find it here :

Menu: Window -> Preferences -> General -> Startup and Shutdown -> Workspaces -> Prompt for workspace on startup

Eclipse preferences has a lot of options. You can search for specific options by typing in the filter text box at the top of the preferences dialog.

If you type in 'startup' for instance the list will be much shorter.

window > preferences > general > startup and shutdown > workspaces

then check 'Prompt for workspace on startup'