To add an external video, the following must be done in BigBlueButton:

First click on the "+" and then "Share external video". Now add the external link.

However, if the video file is stored in Next Cloud there is one more thing to consider!

First click on the "+" and then "Share external video".

You will now be asked to enter the "External video URL" link.

Copy the link of the Next Cloud video and add the following at the end of the link: /download?.mp4

If it is not an .mp4 file but .webm, the ending must be as follows: /download?.webm