Can we create a winforms application which ignores new line charcter in the textbox input and gives a string in single line with a button?

You can do this with a single line of code:

1 set the Multiline, AcceptsTab, and AcceptsReturn properties of textBox1 to 'true.

2 set the Multiline, AcceptsTab, and AcceptsReturn properties of textBox2 to 'false.
note: these are the default values.

3 in a Button 'Click EventHandler:

textBox2.Text = textBox1.Text;

Now, do not be surprised if, when there is no white space at the end of each line in textBox1, what you get in textBox2 is lines-run-together. I'd guess you don't really want to lose all indication of where lines ended ... so:

private Char myLineDelimiter = '|';
private bool doTrimLines = true;

private void MoveTextButton_Click(object sender, EventArgs e)
{
    StringBuilder sb = new StringBuilder();

    foreach (string line in textBox1.Lines)
    {
        sb.Append(doTrimLines ? line.Trim() : line); // see note
        sb.Append(myLineDelimiter);
    }

    textBox2.Text = sb.ToString();
}

Using a 'StringBuilder avoids excess memory consumption that may come into play with manipulating large collections of strings: it's used here out of habit, more than necessity.

Note: do you really want to trim the whitespace in each line ? If you do not trim the lines, and the lines have white-space made of Tabs, those Tabs will appear in textBox2 even though you set its AcceptsTab property to 'false: that's a bug in the TextBox.

See, for instance: .net - Replace Line Breaks in a String C# - Stack Overflow[^].