Available authentification method - API key.
You need to provide X-API-KEY header with every request.
Curl example:
curl --request GET \
--url 'https://api.omnisend.com/v3/contacts?limit=100&offset=0' \
--header 'X-API-KEY: AjWvksZpTG4JZBdc4'
To get your API key go to https://app.omnisend.com/#/my-account/integrations/api-keys