Without a use case I would suggest simply hardcoding this ID value via Set Data Values Extension or if the extension you want the ID from is a JS Code extension add another line that sets it.
If using a Set Data Values Extension (with an ID of 1) and this is the ID you want you can just add another line to the Set rows:
Set: "extension_id" To: Text -> "1" (No Quotes)
This will set extension_id to "1" and you can map "extension_id" to the necessary tag.
If you only want this ID set when a different extension runs, you can add a condition to this extension that checks for the output of the previous extension.
If in a JS Code extension you can simply add:
b.extension_id = "ID of Extension";
Replace "ID of Extension" with the actual ID of the JS Code extension. This will only get set if the JS Code extension runs and again you can map "extension_id" to the necessary tag.