This option should contain the URL prefix, by default this prefix is the current protocol and host such as But some times preview should be passed through a ASPX page or similar and then you want to alter this option.

Variable Substitution

Variable names (including the curly braces) are replaced as follows. Multiple instances of the same variable can be used, and are replaced. preview.urlprefix is currently the only setting in which these variables are replaced.

{proto} Replaced with the protocol of the site (usuallyhttp).
{host} Replaced with the host name of the site (for example,
{port} Replaced with the port number of the site, which is80for most production Web sites.

Example of usage in the config.php file

$mcImageManagerConfig['preview.urlprefix'] = "";

Example of usage in the Web.Config file

<add key="preview.urlprefix" value="" />

Example using variable substitution in the Web.Config file

<add key="preview.urlprefix" value="{proto}://{host}/somepreviewpage.aspx?path=" />

Example of usage in a mc_access file