OSRS
One press should send one ordinary key. Set a pedal to an interface key you picked in OSRS's Keybinding settings (inventory or prayer, for example), or to Space to continue dialogue. Keep clicking on the mouse.
On PCsensor, choose a plain key output, turn off repeat, and avoid text or macro output. On Elgato, use one Hotkey action instead of a Multi Action. This keeps one physical press to one input.
Jagex's rules prohibit hardware that does things you should do yourself. Check Jagex's current rules before using a pedal. See the OSRS foot pedal setup guide for a one-key setup.
Any PC game
Use one pedal for push-to-talk, another for mute, and a third for a clip or scene. PCsensor sends a configured key that Discord reads directly; Elgato can send the same hotkey or use its Discord plugin while the app runs. Pick a key the game does not already use.