Try to use CustomString widget and then create a Source microflow. Add Create variable activity and pass the constant and return the created variable . And you can add some HTML for custom visualisation options.
Since 8.2, you can expose a constant to the UI, and use it in string templates. So, the standard Text widget can show a constant now. Just use it in a parameter.
I tried to use a standard text widget as you described, but I can't give a constant as parameter.I can only give an attribute as a parameter? (see screenshot below)