To edit my book now button:
Open editing
Click the button block.
To change the link click the pencil icon to open the block editor.
Paste in your unique HealthBank URL
Save
Where do I find my unique HealthBank URL?
Go to settings and select listing: https://www.healthbank.io/app/practitioners/settings/listing
More information about squarespace buttons here: https://support.squarespace.com/hc/en-us/articles/206543907-Button-blocks