AutoComplete with String field

[aishwaryamu]

 We have an existing custom table with a set of values. When the user enters data in the field, it needs to check against the value in the existing table and existing column (same like reference field) and if value is not present, it has to save the new value (same like text field)

[Tony Chatfield1]

Hi, unfortunately your post description is not very specific, but it sounds like you need a before insert business rule, that looks up the same table using GlideRecord and if a match is found it aborts the insert setAbortAction(true);

GlideRecord | ServiceNow Developers