I recently wanted to change an investment from default to "dividend reinvestment". The searches and the AI overviews were awful, so i eventually had to find it by trial and error. This is how you do it:

1. Have an account
2. Purchase or transfer securities and wait until it clears
3. Navigate to "Help & Settings" > "Help & Support"
4. Part of the "Investment Settings" on this page is the "Dividend Reinvestment Settings"

There you go. Geez. The instructions were all "go to account and go to dividend investment settings", and that is clearly not the path per above.