Run pgAdmin 4, open the server connection and select the database where you want to use pgagent.

Then open the Tools menu and choose the Query Tool.

Write:

CREATE EXTENSION pgagent;
CREATE LANGUAGE plpgsql;

and press F5 or click on the execute button (the one with the ray icon)

Note: If you get an error saying 'language "plpgsql" already exists' then remove
the CREATE LANGUAGE line and execute the code again with just CREATE EXTENSION pgagent;

Once it's executed successfully, select the server connection again, right click on it and select "Refresh...".

You should see a new "pgAgent Jobs" entry in the database tree in pgAdmin.