I don’t think you can connect the slash-command directly to the gitlab trigger the way you are trying to, because the payload of the slash command is different to what the Gitlab trigger expects. You need to put a small shim in between - an easy way to do this would be either a script behind a webserver in your scripting language of choice, or if you prefer, something like an AWS lambda function, that receives the Slash command from Mattermost, extracts what you need from the payload, and calls to the Gitlab trigger with the correct payload for that.