How do I pass custom field values in CiviReport urls?
http://example.org/civicrm/report/instance/103?reset=1&amp;force=1&amp;custom_65_value=abc should do it.

Note that you can also pass an operator to define the type of filtering you want to do, In this case, for example, adding &amp;custom_65_op=has means that the filter would be 'custom field 65 contains the string 'abc'.

Right clicking on the relevant field in the report and then choosing inspect will open up a window where you can find the appropriate values for different operations.