https://www.reliancecontrols.com/products/reliance-controls-...
Lots of tutorials on YouTube.
It looks like when it's against code it's "by default": if an appliance is not explicitly listed as acceptable for a plug it has to be hard wired. ND and OK have passed state level amendments allowing it, some places have done this locally, and in many places building inspectors allow it with their discretion.
On a beast of a HVAC system, a simple 15A wall plug isn't going to cut it for powering the whole thing, but if the furnace runs off e.g. natural gas and you just want to power the blower off a 15A plug, that solves the problem of switching power sources.
Eg, in North Dakota
"If manufacturer instructions allow the use of not more than six feet of flexible appliance cord, it shall be permitted on permanently installed one hundred twenty volt twenty amp or less fuel-fired furnaces in residential dwellings."
https://www.law.cornell.edu/regulations/north-dakota/N-D-A-C...
For the transfer switch, you can use either a single-pole double throw motor rated switch for a 120V like a Leviton 1285, or a double-pole double throw motor rated switch like a Leviton 1288 for 240V. It must be motor rated.
You’ll also need a power inlet and a cord that connects between your generator and the inlet. Please buy a premade cable and the proper inlet, do not hack together a cord with two male ends aka a suicide cable that you plug into a standard outlet. Example of a safe setup: https://www.vevor.com/power-outlet-box-c_12116/vevor-30-amp-...
Wire the generator inlet to one side of the switch and connect the other side of switch to the utility fed circuit.
Label the switch positions so you don’t forget.
When the power goes out, flip your switch to the generator side, connect your generator to the inlet and turn it on. When the power comes back, flip the switch back to utility and turn off the generator. The center position of the switch is off and it’s impossible to backfeed the utility under normal conditions.