There are two different ways to configure the generated code in this way. The first is block-specific, while the second is a model-wide configuration.
For the block-specific solution, please change the "Output data type" parameter of the "String Constant" block. Instead of using "string", you should be able to change it to "stringtype(n)" where "n" is the maximum length of the string.
For more information on "Simulink Strings", please run the below command in the MATLAB R2018b command window to get the release specific documentation:
web(fullfile(docroot, 'simulink/ug/simulink-strings.html'))
Further, for the model-wide solution, please configure the buffer size of dynamically-sized strings. This can be done by navigating to Model Configuration Parameters > Code Generation > Interface > Advanced Parameters, and changing the "Buffer size of dynamically-sized string (bytes)" parameter. Please run the below command in the MATLAB R2018b command window to get the release specific documentation that discusses more about the "Buffer size of dynamically-sized string (bytes)":
web(fullfile(docroot, 'rtw/ref/buffer-size-of-dynamically-sized-string-bytes.html'))
Please follow the below link to search for the required information regarding the current release:
