Looks like a YouTube bug where JavaScript always interprets a space bar press as play/pause and doesn't disable this action when an input field has focus.

See also:

• /questions/1328819 Youtube search bar issues

A possible workaround for now is to place this character on the clipboard (i.e. type a space and select it and copy the space to the clipboard) and paste the clipboard content when you need to enter this character. You can possibly use the location bar to get this character on the clipboard.

Hi

Thank you for flagging up this issue.

YouTube have looked into this and will be correcting the issue shortly.