dir_path = 'my_project_directory';
file_name = 'test1.txt';
mkdir(dir_path);
out = fullfile(dir_path,file_name)
Note that you do not need to specify any path separator characters as these are inserted by fullfile automatically (and of the correct type to match your OS).
Similarly it is also more reliable to build a filename using sprintf rather than string concatenation, e.g. to generate the above filename:
file_name = sprintf('test%d.txt',1);
