You can't use a Canon extender with any EF-S lens, they aren't incompatible - not all EF lenses will work either - extenders are designed only for specific L-series EF lenses (and a couple of DO ones), one of the reasons they are white. For example, the excellent EF 70-300L lens is not compatible either.
The EF 75-300 lens won't work either and it's optics are woeful to begin with: it would just get that much worse with an extender added if it was physically possible. The extenders are selective on which bodies they will work with too: only a few, recently released bodies will give autofocus and metering options. The ones I know of that give both are the 7DII, 80D, 90D 1DxMkIII.
To do your homework, these links might help.
To expand on Trevor's advice, the M50 is mirrorless, so the lens aperture limits that apply for AF with DSLRs may not apply. There is no mention of it in the camera manual and i know that the R series mirrorless cameras are allowing lenses to be released with f/11 apertures.
Most recent model cameras will do about as well with cropping as they will with a 2X extender in terms of image quality unless you are using very good glass and even then the image quality takes a big hit. I have EF 200 f2 along with EF 300 and 400 f2.8 primes which are some of the sharpest glass Canon offers and I don't care for the results with the 2X using any of these excellent lenses. With lesser glass, the 2X is a disaster.
I think the problem is that the OP wants to shoot long (assumedly in excess of 300mm - 480mm FoV) and has an M-series camera and an EF kit lens. While it is possible to use an EF or EF-S lens with an M-series via the adapter, using them with an extender poses a whole new set of complications. It is for that reason that one tends to go to DSLRs or the R-series MILCs.
Lets' face it, the 75-300 kit lens is a woeful piece of gear and that really comes home when one wants to (attempt to) extend it. As mentioned, Canon extenders don't support it, and while other makers' extenders might, the basic optics of the 75-300 would not handle it.
So, as I see it, if the intent is to get something like decent shots at long range, then one has a few choices:
1. Invest in a good super tele EF mount lens, with an adapter - the following site might offer some choices: Canon EF Mount Lenses - Camera Decision
2. Invest in a Canon EOS 7DII, 80D, or 90D body - the M50 is roughly equivalent to the 80D, and these bodies should be available at a reasonable price as people migrate to R-series bodies. That then allows much more flexibility in terms of optics and no adapter is required.
3. Move to an early R-series body, like the RP and use either an EF lens via adapter or get a lens like the RF 600 or 800 primes at a reasonably cheap price for the reach.
None of these are cheap, but the M50 body was really meant for travel or vlogging (the M5 is more attuned to stills photography) and that is why there is nothing past the 55-200 range in the native EF-M mount.
Very long shooting (500mm and above) requires a bit of an investment. As always, it is hard to offer good advice without knowing the budget and specifics of the needs in terms of results.
I have to admit I have one each 1.4x and 2.0x of the MkIII variety. Specifically for when I could not carry the range of focal lengths I needed. Actually the 1.4x with the 100-400MkII did an OK job, but the 2.0x was only really any good at all with the 70-200 f/2.8 MkII I had.
Absolutely would agree that using a lens with a native FL range for the job is the way to go. Given that, it is almost a relief that one cannot attach one to the 75-300 lenses!
"Teleconverters are one of those things people buy when they first start in photography because they sound like a neat idea."
So true. I would try the cropping route first. Because it is free if you have a simple post editor which most OS's come standard with or d/l one of the free ones. You will see and decide if any extender is worth it with your gear as cropping and tel-cons are about the same in IQ.
So what is your best choice? One of the 150-600mm super zooms either Sigma or Tamron will do nicely.