If you mean saving a text file in OnlineGDB, then you can't as far as I know.
The closest thing you'll get to that, is saving a C file with just a block comment:
/*
This is a C file.
This is used simply to store text.
The slashes (/) and asterisks (*) are used to begin and end the comment.
If you ever put an asterisk next to a slash in your text, it will break.
This is because C can't tell the difference between "a comment with an asterisk+slash" and "a comment followed by code that just so happens to end with another asterisk+slash".
*/