Zoho Sign's API lives at https://sign.zoho.com/api/v1 (sign.zoho.eu, sign.zoho.in for other data centers).
Zoho uses OAuth — create a Self Client in the Zoho API Console with Zoho Sign scopes. Send the token as Authorization: Zoho-oauthtoken <token>. In the examples it's written {{ZOHO_ACCESS_TOKEN}}.
Full reference: https://www.zoho.com/sign/api/
curl https://sign.zoho.com/api/v1/requests \
-H "Authorization: Zoho-oauthtoken {{ZOHO_ACCESS_TOKEN}}"
curl https://sign.zoho.com/api/v1/templates \
-H "Authorization: Zoho-oauthtoken {{ZOHO_ACCESS_TOKEN}}"