Write a single value from 1 row to all rows of another dataset

I have a section of workflow that generates a dataset, each row I will want to POST to an API.

I then have another section that requests an auth token via HTTPCaller and creates a dataset with a single row and single value containing the auth token.

How do I now get the Auth token to set it as a header value in the HTTPCaller when POSTing each row from the dataset?