-D is a command line compiler flag which causes the rest of the text to be treated as if there was a #define in your code.
In solution explorer, right click the project, select "properties". The project property page will open. Expand the ">C/C++" entry in the tree on the left and select "Preprocessor" under that. The top entry in the right pane should be "Preprocessor Definitions". In that edit box, add _SCL_SECURE_NO_WARNINGS, separating it from the other entries with a ;
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