You can add a custom message to your prefilled buttons. We advise you to use the message form field in the snippet. If you change the message in window.SR_SETTINGS make sure to use the backticks (`) and not a normal apostrophe ('). You can simply use 'enter' in this message.
msg: `This is a demo message.
Kind regards,
Your name
`, //(optional) prefill the message
You can create a custom message in your custom button by filling in the message field in the button snippet.
There are 2 options:
1. An existing custom button within a standard object
2. An existing custom button within a custom object
If you haven't made the button yet, you should first create one.
1. An existing custom button within a standard object
Setup > Build > Customize > Your object > Buttons, Links, and Actions
2. An existing custom button within a custom object
Here the objective is to find the button you want to change as well, but the path is a little bit different.
Setup > Build > Create > Objects > Your object > Buttons, Links, and Actions