What is the simplest way to turn on the ESC with PWM input?
No - Almost all hobby grade ESCs require some form of PWM (or digital, for multirotor ESCs) input in order to drive. As mentioned in a comment, you can make a &quot;servo tester&quot; with a 555 timer, or with a microcontroller to generate the waveform.
If you have the information for your ESC, it may be supported by BLHeli, you can modify the source code to handle any input you choose, and reflash the on board microcontroller. BlHeli is open source, so there may already be a branch that handles alternative inputs.
Important to note:

Don't drive the ESC to full throttle unloaded. Timing/field weakening can cause issues/de-sync that may cause damage
Don't do your testing with a propeller on, brushless motors spin quickly and can very rapidly cause damage to the degree of lost fingers.
If the motor is stuttering for a long period of time (more than a second), stop what you're doing an disconnect power. This will damage the ESC if left to continue.