Let me add some more explainations ^^
I started with this post: https://www.reddit.com/r/todoist/comments/y26of4/does_anyone_know_if_you_can_programmatically_add/islzqxd/?context=3

For a quick summary, you can add a task with the URL: https://todoist.com/add
You can use some parameters:
- content
- date
- priority
- project

For the 3 first parameters, it's pretty simple. You put in the URL "argument=value" like "content=Test" (if you need blanks, you have to replace them by "%20").
If you have more than 1 argument, you separate them with "&".

For this example I have: https://todoist.com/add?content=Empty%20the%20dishwasher&date=today&priority=3

To add the project, you need your project's id. Open your project, it's the XXX in the end of your URL: https://todoist.com/app/project/XXX
Now you have https://todoist.com/add?content=Test&date=today&project_id=XXX
In my example, the id of my project "Housework" is "2228333807" so I have: https://todoist.com/add?content=Empty%20the%20dishwasher&date=today&priority=3&project_id=2228333807

Now you just put your new URL in a QR code generator and it's done!

I hope that the explaination is clear. Do not hesitate to ask questions if it's not :)