How do I make a bookmark manager open when Chrome starts?

Enable Caves of Wonders (CoW) as your startup page

During setup, you can enable Caves of Wonders (CoW) as your startup page. You can also open Account Settings, select Preferences, and enable the setting there.

Caves of Wonders (CoW) Preferences page showing the setting to use the bookmark manager as the browser startup page

Open the Cave you need first

Set a Cave as your default by selecting the star button in its toolbar. This setting is local, so different browsers and devices can open different default Caves.

Caves of Wonders (CoW) Cave toolbar showing the star button used to set a default Cave for a browser New Tab page

Keep the Chrome setting

If Chrome asks whether you want to revert the startup setting, select Keep to stop the prompt from appearing again.

Set Caves of Wonders (CoW) as your Chrome New Tab page.