Box allows you to password protect web links, requiring visitors to enter the set password to access the file or folder. This is a premium feature which is unavailable on free personal accounts. To use this feature, click on the little blue box with the white downward arrow next to the file or folder, hover over "Share" and select ‘Get Link to File’ or ‘Get Link to Folder.’
On the pop-up window, open the Access Level menu in the upper right corner, and click "Set Password" (the file has to be Open Access in order to set a password). Enter a password in the text field and choose 'Save.'