How do I send a URL of my video of a run, if I have no YouTube channel? Do I put a video on the forums?

in that case you make a youtube channel and upload your video there

But what if the speedrun is longer then 16 minutes? In that case it is not possible to upload it via youtube.

You simply need to verify your Google account to upload a video longer than 15 minutes: https://support.google.com/youtube/answer/71673