You can do this with a carriage return (aka CR aka character code 13). I am not really clear when that is a \r or ^M, but this worked on a mac using a iTerm. Now, realize that it is not actually the characters ^ (caret) followed by the letter M, that is only how it shows in the terminal. On a Mac I pressed and held ctrl, then pressed V (released) followed by M (released both ctrl and M). This results in the terminal that looka like the two characters ^M, but is actually a single character on the terminal. I know that is confusing, but long and short, it is not the two characters ^M.
Here is an example of how it looks like in the terminal.
admin@pa-220-1# set deviceconfig system login-banner "line 1^Mline 2^Mline 3"
[edit] admin@pa-220-1# show deviceconfig system set deviceconfig system login-banner "line 1 line 2 line 3" [edit] admin@pa-220-1#
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