Access denied due to missing subscription key. Make sure to include subscriptionkey when api request

[Laxmi2024]

I am trying to get Access token through APIM  from third party and have mentioned everything in headers for subscription key, still receiving 401 with this message" Access denied due to missing subscription key. Make sure to include subscription key when making requests to an API."

[Sohail Khilji]

I don't find anything on internet called APIM ? Can you abbreviate APIM ?

[Laxmi2024]

API Manager.