No, that won't work IF the store guy was right. I just worked my way through something like this and it's confusing.
I sounds like the store guy told you the entry fitting is 3/4" NPT (National Pipe Thread). The NPT thing specifies two characteristics: a "standard" thread pitch in threads per inch, and the fact that the threaded end is NOT cut to a straight side exactly parallel to the inner bore. ALL standard pipe threads are done as a shallow CONE - smaller diameter on the END of the pipe, and wider farther from the end. The female fittings also have slightly conical threaded bores. That is why, when you tighten them together, it gets progressively harder to turn farther because the pipe end and the female socket jam into each other to form a tight leak-proof seal. Such connections are made with no gasket, although one does use a thread sealant or Teflon tape to lubricate the turning fittings.
A HOSE outlet is different. It starts out the same - the standard garden hose male and female fittings are 3/4" also! (Hose sizes of 3/8", 1/2", and 3/4" are the inside diameters of the HOSE, not the size of the coupling on the end.) BUT they are NOT the NPT conical or tapered shape, AND they use a "standard" thread pitch DIFFERENT from what a pipe fitting would use. I think they are called GHT (Garden Hose Thread). These hose outlets DO have their threaded surfaces exactly parallel to the bore on the inside, and the end of the fitting is exactly flat with its plane perpendicular to the axis of the pipe or hose. SO the connection is made with a soft rubber WASHER in it that can seal to the two facing flat ends. The turning ring on the female connector and the threads on the male connector screw together to produce compression of the washer between the flat flanges to form a seal. You MAY notice that, if you forget the washer, you CAN screw the two connectors together more that when the washer is present., and that is becasue the fittings are NOT conical and do not jamb together. And of course, if you forget the washer, you can never get a leak-proof connection.
The standard outlet fitting for a garden hose is the hose BIB, a name for a particular tap whose body has a threaded output of exactly the right diameter and thread specs for garden hose female connectors. If you somehow decided to rig up your own hose outlet from a short piece of 3/4" NPT pipe, you would find you can NOT screw the hose female connector on it. And that is why I said the adapter suggested above will not work. It is made with both ends as standard HOSE female fittings, so it will NOT fit onto the end of a 3/4" NPT pipe.
NOW my question is: did the store guy know all this? Was he correct in calling the inlet a 3/4" NPT thread? They do look a lot alike! So do a simple test. First, try to screw a real HOSE female connector onto that pipe WITHOUT a washer. If it will screw on smoothly all the way until the hose end hits the pipe end, then it IS a standard 3/4" HOSE male fitting. And in that case, the adapter linked above WILL be your right answer, because it can change a male on the end of your hose to a female that fits the washer inlet. On the other hand, if that does not go smoothly, get a standard 3/4" NPT pipe female fitting (like an elbow) and try screwing that on. If it goes smoothly, then tightens up as you go further, then the washer inlet pipe really is a 3/4" NPT. In that case, you need a plumbing shop to find or make an adapter that can screw onto a 3/4" NPT male pipe end and convert it to a FEMALE standard 3/4" Garden Hose fitting (just like the inlet of a hose nozzle) that you can screw onto the end of your hose. Now, even that can cause you a problem. It's easy to screw a nozzle onto the end of a hose. But imagine trying to screw a POWER WASHER onto the end of your hose! So the FEMALE input end of this adapter NEEDS to be a freely-rotating ring , such as you often find on a larger sprinkler, and not just a fixed part of a nozzle casting.
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