Querying records where string length is less than N

I have a table with US Zip codes where some zips with leading zeros were not imported properly (the data type was originally integer) when importing from Airtable.

I would like to prefix these zip codes with a '0'. However, upon looking at Querying All Record function, I can't seem to add a Strlen filter in the WHERE clause on the field.

In Xano, I wanted to do this:
1. Query all records where the length of the postal_code is less than 5
2. for each record found, prepend a '0'

In SQL Server, I can do something like this:

update US_Zip
set postal_code = '0' + postal_code
where len(postal_code) < 5