How to backup Macbook pro to icloud now?

Apple offers iCloud Backup for iPhones and iPads but not for Mac computers.

It is possible to sync data from a Mac to iCloud if you turn on those options (e.g., iCloud Photos, iCloud Drive including the Desktop & Documents option). Syncing has some of the benefits of a backup (and the benefit of being offsite) but is not the same as a backup. Major differences include the fact that not everything on your Mac can be synced and the fact that if you delete something, accidentally or otherwise, there is only a limited time period (30 days in most cases) during which it can be recovered.

Also see: https://support.apple.com/mac-backup

And more information on syncing in:

https://support.apple.com/en-us/HT204264

and

https://support.apple.com/en-us/HT201104

"On the cloud" is great for sharing photos, but is not a viable backup solution for everything you have. The stuff is not under your control, and is subject to sloppy handling, arbitrary changes in policy, theft, data loss, and bankruptcy of the company that holds it. It can easily take three days to restore it at ordinary Internet speeds.

If you do not have a recent local, disk-based backup, your computer is like a ticking Time bomb. You are only one disk failure away from losing EVERYTHING! Drives do not last forever. It is not a question of IF it will fail, only WHEN it will fail.

If you are using another direct-to-disk backup method that you prefer, and you currently have a recent disk-based backup, that is great. If not, you should consider using Built-in Time Machine. Take steps to acquire an external drive as soon as possible. If you buy one, a drive 2 to 3 times or larger than your boot drive is preferable for long term trouble-free operation. Do not pay extra for a drive that is fast.  (You can get by for a while with a "found" smaller drive if necessary, but it will eventually become annoying).

Attach your external drive and use

System preferences > Time machine ...

... to turn on Time Machine and specify what drive to store your Backups on.  It may ask to initialize the new drive, and that is as expected.