Illustrated in the below image is an example of the Record Generator step as a primary source step and an HTTP Service step as the secondary source in a sub-flow.
NB: “{@inputRecord.fieldname}” is used to pass a parent record for subflows and other control steps.
To pass a field from the main flow to a sub-flow, “{@inputRecord.fieldname}” is used. In this example, it is being passed as a header value in the HTTP Service source step as shown in the below example.