I have recently placed an order for this bike rack and if you scroll down to the Features section you will see it says it requires the following type of hitch receiver:
Hitch Size: 2” only (Do not use a 1.25”-2” hitch adapter)
From UHaul's list of available hitch receivers here: https://www.uhaul.com/MovingSupplies/Hitch-Receivers/
which one should I get that will work with the bike rack I placed an order for?
As Daniel R Hicks writes, any sturdy 2" receiver hitch should work, ** as long as it fits your car **. Hitches are usually specific for one or a few car models.
Hitches are designated according to the width of the square receiver opening. 1.25" hitches are typically lighter than 2" hitches, but are also less robust and can't support as much weight. Since you are getting a heavy-duty rack to carry heavy e-bikes (80 lbs/bike max vs. 35 lbs/bike max on a "normal" rack), you need the strength and weight capacity of a 2" receiver hitch.
As mattnz notes, a bike rack slotted directly into a hitch receiver will generate higher torque forces on the pitch axis (rotating forward/back) than would be the case with a standard tow ball/trailer combination. The hitch needs to be even beefier to withstand these higher torque forces.
The hitch specifications will probably indicate a maximum "tongue weight" which is the maximum weight pressing down on the end of the hitch that it is rated to hold up. The larger the tongue weight rating, the better, as the weight of the rack and bikes will essentially be tongue weight.
Your UHaul link lists numerous "DrawTite" and "Curt" branded receiver hitches that are noted to be 2" in the hyperlink wording of the specific hitch. As noted in the other answer the key to selecting the correct 2 inch receiver hitch is determining which one will fit your vehicle it's to be mounted on. There are 5 classes of receiver hitches, and a hitch is classed according to it's rated towing capacity with class one being rated for the lowest amount of weight that can be towed (2000# max) up to Class 5 (20,000# max).
The significant factor in this case is your bike rack's requirement for a 2" receiver hitch. Two inch receiver hitches are only found on class 3 and above receiver hitches. Class 3 receiver hitches are the most common hitches found on full sized pick-up trucks and SUVs sold with a "tow package" owing to their versatility and tow capacity of around 8000#. Class 3 hitches can be installed on full sized vans and some large crossovers.
There are some manufacturers that make custom hitches that may or may not fall into the 5 classes. Curt Manufacturing is one such company from Wisconsin, USA. This is not an endorsement of Curt Mfg, but rather the one company I'm familiar with that offers custom hitches which can be mounted on a car and also have the required 2" receiver tube you require. If it is a car you plan on using the bike rack for, another option is a bumper hitch. These are smaller, less expensive than larger "classed" hitches, and can come with the desired receiver tube size of 2" (not all bumper hitches do and many bumper hitches aren't receiver type hitches). Bumper hitches have a relatively low tow capacity of less than a 1000 pounds, but if the bike rack is what you'll use it for, tow weight capacity is irrelevant.
Online and brick and mortar sellers of hitches and accessories will have some sort of system that allows you to enter your vehicle's year, make and model (and your requirement of a 2" receiver) to generate a list of hitches that will work for you. As a specific response to your question: In your UHaul link that lists available hitches, you'll have to select one of the class 3 hitches that work with your vehicle as they will have the 2" required receiver, but your vehicle should be a full sized van, SUV, or pick-up to utilize these. Other options mentioned above need to be explored if it's a car you'll be using for the hitch and bike rack.
