Don’t
It’s idle and lazy and at some point some poor sod is going to have to dig it out.
We built our house 10 years ago and as I was working 200 miles away during the week the builder put in the gate posts 3000mm 200mm by 200mm buried 31/2 feet. He poured postcrete to a depth of that 3.5ft and ft deep and my wife and dipstick father in law thought that “marvellous”.
Growing up in a farm we never used it - we put gate stoops in deep but wedged with stones. It takes longer and it’s harder work but the point is when you need to get em out you can. It’s called doing the job properly.
Because our drive now backs directly to the shared block pave access drive our neighbour put in after the gate posts went in I’m now having to chip through 3.5 ft of postcrete on two sides and then chisel the stump of the lousy quality stoops the builder procured.
Postcrete - a lazy solution to fixing stoops - do it properly - dig it deep and wedge it when you put the post in.
Rant over