First of all. Welcome. This is what my screen shows above your post and is so cool.
Secondly, to answer your question, I'm almost certain you'll have to do it for each of them in app, but i do have a hacky way to do it since you're on Linux. You should have vim installed on your system. These steps should work.
Install vim if not already available.
Open a terminal
Look for what looks like a box with an arrow pointing up and right on your toolbar in Joplin. This should force Joplin to open a Vim terminal instance that can be used to edit your note.
Make your way to the beginning of the second item on your list with your keyboard keys.
Hit Ctrl+V to go into Visual Block Mode.
Use your keyboard to move the cursor to the beginning of the last item.
IMPORTANT STEP: Hit Shift+i to go into Insert Mode.
Type - (single hyphen) and then Escape. Your list items should each have a hyphen with a space in front of them.
Type :wqa to exit Vim and save your note to Joplin with every item formatted properly in a list format.