Looks like 3/4" thread x slip. Can drill an access and probably heat a barb or epoxy one in place as mentioned above.
If it's 1/2 (which I would have to measure mine to see when I get near it again) as @S-Met says, you'll have to dig some more, but I was able to reasonably quickly google and find drip irrigation part that's 1/2 with barb
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