How to obtain an access token for accessing LTI Advantage Services?

I am receiving the following error:
{"error":"invalid_client","error_description":"Missing client"}
My curl headers are set:
--request POST
--header "Authorization: Basic B64 Encoded clientID:client secret (this is missing!)"
--header "Content-Type: application/x-www-form-urlencoded"
My URL is set:
https://auth.brightspace.com/core/connect/token?grant_type=client_credentials&client_assertion_type=urn:ietf:params:oauth:client-assertion-type:jwt-bearer&scope=https://purl.imsglobal.org/spec/lti-ags/scope/score https://purl.imsglobal.org/spec/lti-nrps/scope/contextmembership.readonly https://purl.imsglobal.org/spec/lti-ags/scope/lineitem&client_assertion=JWTGOESHERE&redirect_uri=https://testing.com
When registering an LTI Advantage Tool, I cannot find the client secret. It only provides the following registration details:
- Client Id
- Brightspace Keyset URL
- Brightspace OAuth2 Access Token URL
- OpenId Connect Authentication Endpoint
- Brightspace OAuth2 Audience
- Issuer
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