Use "Perform Find" script steps and open "Specify find requests"
click on "New"
"Find Records When" - select the relationship for you portal and your filed
Add Criteria (could be variable) and click on Add button on the right
"Enter Find Mode" script step
"Set Field" - set to a search value the field from your portal
Perform Find with unchecked "specify find requests"
But keep in mind that "context" makes a big difference here. When you specify find criteria in a field from a related table, such as a field in a portal, you are NOT finding records in that related table. You are still finding records in the layout's table. You are telling FileMaker, "Find all records in the layout's table with at least one related record matching this criteria."
Once the find is performed, the layout returns to browse mode and the portal displays all related records--both those that match your find criteria and those that don't. So if you are looking for records in the portal's table rather than the layout's, your script has to change layouts to a layout based on the portal table.
You also may find this thread of scripted find examples helpful: Scripted Find Examples