Adding text to beginning of text field using "custom format script"

Hello, I am in the process of making an editable PDF that includes fillable text fields. I have a text field for a certificate number. My intention is to habe "Cert No:" to the start of that field, allowing the user to simply input the number while ensuring that the phrase "Cert No:" always remains at the beginning of the field.

I have encountered scripts that allow for placeholder text to vanish once the user begins typing in the field; however, I am seeking a script that ensures the words remain visible and always at the start of the field.

Thanks.