Datadog's API lives at https://api.datadoghq.com for the US1 site; other sites use their own host (e.g. api.datadoghq.eu). Most reads need both an API key and an application key.
Create an API key and an application key under Organization Settings. Send them as DD-API-KEY and DD-APPLICATION-KEY headers. In the examples they're written {{DD_API_KEY}} and {{DD_APP_KEY}}.
Full reference: https://docs.datadoghq.com/api/latest/
curl https://api.datadoghq.com/api/v1/validate \
-H "DD-API-KEY: {{DD_API_KEY}}"
curl "https://api.datadoghq.com/api/v1/monitor?page_size=5" \
-H "DD-API-KEY: {{DD_API_KEY}}" \
-H "DD-APPLICATION-KEY: {{DD_APP_KEY}}"
type 3 is a gauge. timestamp is Unix seconds within the last 10 minutes.
curl -X POST https://api.datadoghq.com/api/v2/series \
-H "DD-API-KEY: {{DD_API_KEY}}" \
-H "Content-Type: application/json" \
-d '{"series": [{"metric": "posttaco.test", "type": 3, "points": [{"timestamp": {{UNIX_TIMESTAMP}}, "value": 1}]}]}'