You set up a custom dimension in Google Analytics. It is not showing up as a choice in the dimensions drop-down box in the Fivetran Google Analytics Connector's setup form.
Connector: Google Analytics
The custom dimension will show up as
dimensionXX in the Fivetran Google Analytics Connector setup form.
The value of the requested custom dimension is stored as
ga:dimensionXX, where XX refers to the number or index of the custom dimension.