Either construct the String by concatenating together the elements you want and then use it or use the elements separately
Personally I have used C style strings (NOTE lowercase s) and used the snprintf() function to replace the value of the %s format character with the string required
It's just a value sent when the button is pressed. This part of the code works ok.
Please see the end of the button line where I'm trying to inject String - usuccessfully.
You need 3 Strings as in my previous example
One with all the HTML code before the button text
One with the button text, which may change
One with all the HTML text after the button text
Then you concatenate them together as in my example
I note, however, that you appear to have quotation marks in the button class String and these will need to be escaped using the \ character as otherwise the compiler will see them as the end of the String and an error will occur