HCP Terraform's API at https://app.terraform.io/api/v2 follows JSON:API: send Content-Type: application/vnd.api+json on writes.
Create a token at User Settings → Tokens. Send it as Authorization: Bearer <token>. In the examples it's written {{TFC_TOKEN}}.
Full reference: https://developer.hashicorp.com/terraform/cloud-docs/api-docs
curl https://app.terraform.io/api/v2/account/details \
-H "Authorization: Bearer {{TFC_TOKEN}}"
curl https://app.terraform.io/api/v2/organizations \
-H "Authorization: Bearer {{TFC_TOKEN}}"
curl https://app.terraform.io/api/v2/organizations/{{ORG_NAME}}/workspaces \
-H "Authorization: Bearer {{TFC_TOKEN}}"