How can I search for two fields using only
one select field?

If I look for one, I do it like this:
&build_select_field ("Field","$rec{'Field'}")

but I want to search the same value also
from field "Field2".

You'll either have to have a second select field for Field2 or you can set the name of the select field to "keyword." If you do the latter, it will look for the search term in every field, but you won't be able to use &build_select_field. You'll have to build the keyword select field yourself.

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JPD