how do I connect a pc laptop to use the iMac display as a secondary screen

You don't. Target Display Mode was never supported nor documented by Apple for non-Mac products. Also, TDM has now been fully deprecated, finishing a process Apple started six years ago.

Only a small sub-set of iMacs ever worked as external monitors for another Mac. They were made between Late 2009 and Mid 2014. Even the support article has been archived.

Use your iMac as a display with target display mode - Apple Support

Followup: I believe DBAqua has asked for that additional information because, totally by accident, someone found that 27-inch 2009 and 2019 iMacs would work with some non-Apple devices. No other iMac would do that.

This was never officially supported or documented by Apple so people, were on their own. However, if your 27-inch iMac is 2009 or 2010 and your PC ports are the right ones, it may work.

Which exact model of iMac & what ports does the PC have?