You'll need to use the user's Account name and not their User name for this. Account names are specified in Manage | Security. User names are specified in preferences.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
And note that any find performed once you have this set up will automatically omit records for which the current user does not have access permission.
Thank you Phil, my confusion came from having multiple portals on the layout already, but I think if I add a field with their Account name into the table then I can just filter all the portals that way.