lines of a textbox

[MarinoRules]

how does one specify which line of a text box they want to add to?

[plenderj]

Well you cant really specify which line.
You could however do as in the example below...

Add two textboxes : Text1 and Text2.
Set their MultiLine Properties to True.

Option Explicit
Private tempArray() As String

Private Sub Form_Load()
    
    Text1.Text = ""
    Text2.Text = ""
    
    Me.Show
    Text1.Text = "This is the first line" & vbCrLf & "This is the second line" & vbCrLf & "This is the third line"
    tempArray() = Split(Text1.Text, vbCrLf)
    
    'Each line of text is now stored in an array :
    'tempArray(0) == "This is the first line"
    'tempArray(1) == "This is the second line"
    'tempArray(2) == "This is the third line"
    
    Dim tempStr As String
    tempStr = InputBox("Which line would you like to add ?" & vbCrLf & vbCrLf & "1  ,  2  or  3")
    If (tempStr = "1") Or (tempStr = "2") Or (tempStr = "3") Then
        Text2.Text = tempArray(tempStr - 1)
    End If
    
End Sub