Setting aws sessionToken in AWS signature v4

Using the node.js library - I need to set the sessionToken on credentials for my S3 put command to work with proper permissions. This is how I would set the credentials directly.

  AWS.config.update({accessKeyId : credentials.AccessKeyId,
    secretAccessKey:credentials.SecretAccessKey, 
    sessionToken: credentials.SessionToken});

Trying to re-create the same upload s3 request using the REST api only, I've used the aws4 library to generate the AWS signature v4 but I don't see any place to set the session token.

Making the call to my s3 put command without the sessionToken using the AWS.S3 object throws a permissions error - and the same as well with the REST api.

How can I set the session token into the v4 signature?