find name and add sentence in C# (windows form) : I have a name List (name.txt) and want if find "David" name , A line before add "find my name"

For example:
Name.txt:
<br>
Jacob
<br>
William
<br>
Ethan
<br>
James
<br>
**David**

--------------
i want = 
<br>
Jacob
<br>
William
<br>
Ethan
<br>
James
<br>
**"find my word"**
<br>
**David**

tip: David name may have been repeated many times!
I want Repeat "find my word"

my code:

        private void button1_Click(object sender, EventArgs e)
        {
            if (File.Exists("name.txt"))
            {
                string content = File.ReadAllText("name.txt");
                string sensentence = "find my name"; 
                //I don't know how to proceed
            }
        }


Thanks Guys