When my buddy, who had a hobby as a reptile breeder, decided he needed generator power backup in a rented house, he chose to bring emergency power into the main breaker panel using the popular but unsanctioned method of a cheater cord into the dryer outlet.

The reptiles needed warmth to stay alive, and his collection was worth tens of thousands.  Because he had several rooms full of cords feeding low wattage tank heaters, and also wanted to supply the usual refridgerator and furnace loads, the extension cord method was a real nightmare, would require a big investment in cords and splitters, and hours to setup and break down.

Dryer Outlets are attractive for this purpose because they are high rated, fed via large gauge wire and generally located close to the service panel.   My friend's house was equipped with an older 3 prong, 30A outlet.  I built him a cheater cord that came out of the generator and plugged into the dryer outlet, male connections on both ends.  This is unsafe if plugged into a live circuit on one end, and left unplugged on the other end, which is just one reason why the practice is unapproved, but he wasn't interested in spending a lot of money upgrading the landlord's property, with a proper interlock, so took the expedient path.

It's essential to shut off the Mains, isolating the breaker box from the grid.  Also essential to shut off any 240 Volt loads, since they would be fed single phase during the emergency event.  Some generators can supply split phase 240V, but not the one he was using.  It is possible to make a cheater cord that will supply the same single phase to both legs of the split phase circuit breaker box, and this is a viable strategy if one is disciplined about isolation.

The last essential step is to identify which circuits are essential, such that the generator, dryer outlet, and inlet cord won't be overwhelmed.  We used colored tape to ID circuits which must be OFF in red, and those to turn ON in blue.  Anything left unmarked was safe to turn on, but not essential.

It's a practice that is a half measure, and not by any means idiot proof or without risk.  But, it does work, and requires no modification to a building's electrical system to implement.  The only evidence of shennanigans is that cheater cord, which can be quite short.