php - Sending an envelope: getting 301 response code back

PHP 7.4, Windows Server 2012, IIS8

I'm using the DocuSign PHP SDK method createEnvelope, which sends the envelope object to their server. The URL is

https://demo.docusign.net/restapi/v2.1/accounts/cheese-burger/envelopes

with these headers

X-DocuSign-SDK: PHP
Authorization: Bearer ee-i-ee-i-oh
Accept: application/json
Content-Type: application/json

I'm successfully using the JWT key for, say, getUserInfo, but when I submit the envelope I get

HTTP/1.0 301 Moved Permanently

back. As far as I can tell, the envelope is created correctly, I know the JWT key works, and account id is correct. Any insight as to why I'm getting a 301 redirect?

Solution:

@matt clark supplied the answer in the comments--the 301 response header helped the OP realize that he was missing the protocol section of the URL.