Retreive your optiseller keys as per https://help.optiseller.com/en/articles/8255236-getting-started-with-the-optiseller-api

The developer portal link is also located here which is at https://developer.optiseller.com/

In the developer portal, you can explore and test the APIs, by clicking the "Try It" button:

This opens a form to allow you to enter the parameters, and if you copy and paste the subscription key form the portal, you can call the API immediately

In this example, we've used the BrandMPN parameter to try and source Item Specifics via teh "GetAspectSuggester" API

And the results are shown at the bottom of the form