Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
The formatting of your example looks correct for the GET list membership request.
https://v3.developer.constantcontact.com/api_guide/lists_overview.html#example-request---get-list-membership
If you’re getting an error, your token may be expired, as they only stay active for a limited amount of time.
https://developer.constantcontact.com/api_guide/auth_overview.html#tokens
I’d recommend refreshing your access token and then using the new token value in your request:
https://v3.developer.constantcontact.com/api_guide/server_flow.html#step-4-exchange-the-authorization-code-for-an-access-token-and-a
... View more
