By default you can't do it. You'd need to either:
- create a plugin to add that functionality
- use alternative sanitization (install nodebb-plugin-sanitizehtml and disable html sanitization in markdown plugin).
The second way is much easier, but due to the html parser used it has its own problems (for example if you ever want to write that something is less than or equal something, you can't use
x<=y, because it will think you started a html tag and will cut out your post until
> is found or until the end. It's not really fixable unless sanitize-html changes its parser to something like parse5... It can only be worked around manually by modifying the plugin).
Markdown doesn't have a line break tag other than actual line ending, so because it's basing the parsing on line endings you can't have more than 1 line in a cell.
EDIT: Markdown actually has a line break - but since NodeBB treats soft line breaks as new lines and not spaces, it's not really useful here usually. However, ending a line with a \ and a single enter (or 2+ spaces, but a backslash is more visual) will create a hard line break.
Perhaps making it work without disrupting a table cell would actually be a good idea? It'll make them harder to write, as you'll have to do something like:
| first line of fists cell\
second line of first cell | first line of second cell\
second line of second cell | first line of third cell |
But it's much better than nothing.