I think when you use multiline then the text that you want to type in
th text field wraps automatically to the available line length.

Sol Apache was telling us:
Sol Apache nous racontait que :

Yes.

How are you inserting this second line?

Three ways:
1) While in the VBA editor, select your textbox. Wait a few seconds and
click on it again (Just make sure you do not do an actual double-click
because this will take you to the code pane with a Click event sub), you
should be able to type the default text directly in the text box instead of
going through the properties pane. Now, as you type, if, as you say,
MultiLine is set to True, the text will automatically wrap to the next line
when you get to the end. You can force a new line by hitting  CTRL-Enter

2) Just type the text in the Text property in the properties pane, if it is
too long, it will automatically wrap.

3) Use something like this in the Initialize event:

   Private Sub UserForm_Initialize()

   Me.TextBox1.Text = "This is the default text that should be" & vbCrLf &
"on many lines becasue it is too long."

   End Sub

Here I used a vbCrLf to force a nee line, but you could remove " & vbCrLf &
" and replace it by a single space and the text would automatically wrap.