Make your first call

Create your Client ID
Log in here to Dev Console to get a ClientID and your secret key. To learn moe about creating client_id and secret_key, read here.

Get an Access Token

Make a call to to the URL https://api.sandbox.interswitchng.com/v1/oauth2/token. Pass the client_id and secret_key in the Authorization header. This follows the standard Basic Authorization realm:
Authorization: Base64(client_id:secret_key)

Programmatically, it will look like:
String authorizationString = client_id + secret_key;
String authorizationBase64 = Base64.encode(authorizationString);
HTTP.setHeader(“Authorization”,  “Basic ” + authorizationBase64);

The body of the request should be:
grant_type = client_credentials

Request
Making a CURL call, they snippet below will suffice:

curl https://api.sandbox.interswitchng.com/v1/oauth2/token \
-H “Accept: application/json”  \
-H “Authentication: Basic bXktdHJ1c3RlZC1jbGllbnQtd2l0aC1zZWNyZXQ6c29tZXNlY3JldA==”  \
-d “grant_type=client_credentials”
The resultant network packet should look like:
POST / v1/oauth2/token HTTP/1.1
Authorization: Basic bXktdHJ1c3RlZC1jbGllbnQtd2l0aC1zZWNyZXQ6c29tZXNlY3JldA==
Content-Type: application/json

grant_type=client_credentialsResponse
{
“access_token” : “c4cf8b16-7643-4cea-821f-73e5fe97fa19”,
“token_type” : “Bearer”,
“expires_in” : “3600”
}