how do I back up to time machine with time capsule?

LauraSlaney wrote:

how do I back up to time machine with time capsule?

Say what?

Get a second drive and start a new backup of Time Machine if you want two copies of TM

see "Select your storage device in Time Machine settings"

ref: Back up your Mac with Time Machine - Apple Support

If you value your user data

3-2-1 Backup Strategy: three copies of your data, two different methods, and one offsite.

More than one device, more than one backup methodology.

2016, Apple disbanded the wireless router team that developed the AirPort Time Capsule and AirPort Extreme router. 2018 Apple formally discontinued both products, exiting the router market.

https://www.macworld.com/article/231973/switch-away-from-time-capsule-to-time-machine-volumes.html