You probably can't add an arbitrary line break.
However, you can influence where a line break occurs if the text is to be rendered inside a container that is not long enough to display the text as a single line. Just use non-breaking spaces at the right places in the text.
Let's suppose the text contains sentences and it is going to be broken to several lines and you want a line break to occur before a certain sentence. Then use non-breaking spaces instead of regular spaces in the sentence. The specific sentence will then possibly not fit on the current line and will be rendered on a new line:
The text is to be rendered in this box. Let's force a line break now.
This•text•starts•on•a•new•line as it will not fit on the previous line without breaking the text
(• stands for ).