Splunk Add-On Builder Pre-Request Script

In order to hit the endpoint needed to get data from an external API, I need to make a call to get a valid access token from the external API. I use that access token to make subsequent calls to the API, specifically the endpoint needed for data input into my Splunk Add-On. My understanding is that when using REST for the data input method, you are only able to use a Global Account, passed into the headers, to do authenticated calls. I do not see a way to do a pre-request script in order to get the access token prior to making the subsequent call. Is this possible in a different way?