Create a newline character with newline and concatenate it onto a character vector.

chr = 'Whose woods these are I think I know.';
chr = [chr newline 'His house is in the village though']

chr = 
    'Whose woods these are I think I know.
     His house is in the village though'

Although chr displays on two lines, chr is a 1-by-73 character vector that contains the two sentences, separated by a newline.

Starting in R2017a, you can create strings using double quotes. Create a newline character. Then use + to concatenate the newline character and more text onto the end of a string.

str = "In Xanadu did Kubla Khan";
str = str + newline + "A stately pleasure-dome decree"

str = 
    "In Xanadu did Kubla Khan
     A stately pleasure-dome decree"

Although str displays on two lines, str is a 1-by-1 string.