Passing a string, around quotation marks into another string, all in a mutli line string
No matter how you do it, it'll be always messy. Instead of hardcoding the strings use resource files instead. You can embed *.txt files inside the exe/dll and easily read them without having to escape anything.

See this question on Stack Overflow: How to read embedded resource text file
. You'll find everything you need there.

To insert the message use the {0} placeholder with

string.Format(stringFromResource, txtMessage.Text);