Confluence Cloud's REST API v2 lives at https://<site>.atlassian.net/wiki/api/v2.
Basic auth with your Atlassian email and an API token — curl's -u, PostTaco's Auth tab. Create the token at id.atlassian.com. {{ATLASSIAN_SITE}} is the part before .atlassian.net.
Full reference: https://developer.atlassian.com/cloud/confluence/rest/v2/intro/
curl https://{{ATLASSIAN_SITE}}.atlassian.net/wiki/api/v2/spaces \
-u {{ATLASSIAN_EMAIL}}:{{ATLASSIAN_API_TOKEN}}
curl "https://{{ATLASSIAN_SITE}}.atlassian.net/wiki/api/v2/pages?limit=10" \
-u {{ATLASSIAN_EMAIL}}:{{ATLASSIAN_API_TOKEN}}