Unable to update field data value from one table to another table

Hi all,

I have extended my Purchase Header table and Page to include Job No. and Job Description fields and what I need is when I pick the Job No on the Purchase Header page, my custom fields on Job page should be updated the vendor no and Vendor name from the Purchase order. I have put all my code inside OnValidate trigger but immediately I pick the job no and description on my Purchase Order page and save, my job card window fields are not updated. Below is the extract of my code.