You have the ability to write a shell script on the “Pre-Data” event. See event [ 2a ] in our table of events that Templater broadcasts. In that script, you can run any commands needed to retrieve an API access token. Does this help?
For remote URL data sources, Templater supports Basic Authentication at the moment. How are you expecting to use the API access token once you retrieve it? Are you wanting to send it with request Templater makes? If so, I’m not sure that’s possible out of the box even with scripting. However, you should be able to get the API token via a shell script by registering it to the “Pre-Data” event.
Also, don’t forget that you may need to enable the “Tab delimited file exported from Excel or Numbers” checkbox within the Templater Preferences dialog to work with TSV data exported or saved from Microsoft Excel.