Really depends on your current tool so RTFM on that.
But when you are activating it in your account? There is a QR code you are supposed to scan. And there is almost always a button like “Having trouble?” or “Show TOTP Key” or whatever. Click that and you get a long alphanumeric string instead. Paste that into the TOTP field for Bitwarden (or Keepass or whatever) and it will generate codes for you.
Once or twice I have had to actually use my phone camera to decode the QR code so that I can manually type in the TOTP code/seed, but I think the last time I did that was in like 2020?