SetEnv sets a particular variable to some value, so you need something like
SetEnv varname varvalue
If this is for a specific virtual host, and you have access to the Apache configuration files, this would go inside the <VirtualHost> directive for that virtual host.
If you don’t have control of the config files, you’ll need to put it in a .htaccess file. But for this to work, SetEnv must be allowed in .htaccess files, which is specified using the AllowOverride directive.