Whether it be to do your job, get your schoolwork done, clean your house, work on your creative passion, etc.
Bonus points if you’re someone without reasonable self-control so saying something like “I can only have a cookie after I do this/while I do this” doesn’t work for you and you’re just going to eat the cookie and not do it.
I don’t think, I just do.
I usually keep an end goal in sight; for example, I sometimes dread going to the gym, but I always remember that my goal is to stay fit, have a healthy body and exercising is an important part of that. By sticking to my goal, I maintain my discipline and go to the gym 3 times per week.
Another example is school homework; in my case, my math homework is something I don’t enjoy, but I remind myself that I need the math certificate in order to enroll for a Computer Science degree at an university. Therefore, I keep pushing myself to study math and get good grades (which so far has worked pretty well)
“I don’t want to get fired and subsequently lose my healthcare and housing” isn’t a good motivator though. It only causes dread.
It’s also different with ADHD because everything in your body actively tells you to stop doing the thing and do something else. If you do the thing, you use up your entire supply of spoons for the day and barely have enough to get home.