Plugin User Input window

Is there a way for an addon/plugin to open a user input window after certain event triggers?

It depends on the event. If the event is propagated you can trigger your own GUI. I would recommend using qt, as TS itself is built upon it.

I am kinda new to ts3 plugins, but I assume the main function just has listeners that open the according event functions whenever they are triggered. I now want the program to run some code when a certain time is reached. Therefore I am trying to have a function that is comparing this previously set time to the system-time. Is there a way to make such a loop or event that triggers every 10ms to compare these 2 times?

Ok, so first you have to realize that all functions within the plugin are called synced (blocking). Thus you will have to open a new thread / process to handle your own loop.
All callback functions will run in the main thread so will will have to implement some cross-process communication with either atomic variables or blocking structures.

You can find the sdk with some example project here:

I would recommend starting your process in the init function: