First, you should read what Discord says about Slash Commands.
In March 2021 Discord introduced slash commands. Task Manager also supports them.
How to use
In order to use slash commands, the bot needs permission to create slash commands on your server. If you aren't sure if the bot has this permission you can visit the invite page and just add it to your server without kicking him before. The permission will update and the Bot can create slash commands.
When you type in a "/" (slash) in the text input field new options pop up.
If the commands of Task Manager aren't shown directly, you can tap the profile picture of it on the left of the popup.
There you can see a variety of commands. Every command has a little description. If you can't find the command by scrolling, start typing it. If the command exists it will get displayed.
Discord guides you through all the parameters required to perform the command.