Tilde ~ is supposed to open the autocomplete for channels and link them.
On a Belgian keyboard, you have to press Alt-Gr = twice to get a tilde, but that only opens the shortcut help.
Here is the Belgian layout as a reference : https://upload.wikimedia.org/wikipedia/commons/9/93/Belgian_keyboard_layout.png
Steps to reproduce
The version of Mattermost is the last.
To reproduce I guess changing your layout to BE and hitting Alt-Gt+/ on a US keyboard should do the trick
Does anybody have an idea how to get around that ?
The best solution I have for now is to write the tilde by doing Alt+126