A question about trigger fucntion syntax

Good morning (at least is is morning East Coast USA time).

I am trying to create a function to validate an attempted record
insert, and I am having a hard time coming up with syntax that
is acceptable.

Here is the scenario I have a table that has (among other items) employee_key
and work_type_key (both integer FOREIGN KEYS). Then I have another table
that has the following structure:

CREATE TABLE permitted_work (
employee_key integer ,
work_type_key integer ,
permit boolean DEFAULT FALSE NOT NULL ,
modtime timestamptz DEFAULT current_timestamp ,
FOREIGN KEY (employee_key) references
employee(employee_key) ,
FOREIGN KEY (work_type_key) references
work_type(work_type_key) ,
CONSTRAINT permit_constraint UNIQUE
(employee_key , work_type_key)
);

What I think I need to do is create a function that is fired on an insert,
or update to the 1st table that verifies that there is an existing row in
permitted_work that matches the combination of employee_key AND
work_type_key AND has the value TRUE in the permit column.

First does this seem to be a good way to achieve this constraint? If not,
I am open to suggestions as to other ways to address this requirement.

If it does, could someone give me a little help with th syntax of the
needed function ??

Thanks for your time helping me with this.

--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin