Trusted Save As function from text fields

Hi everyone, I have finished all of my other PDF based projects with great help from the community. I am now at the final task which as the name of the topic suggests, is a save as function. I have read the documents about it, I am not fully understanding.

Can someone show me what the script is supposed to look like for a save as function that calls from three text fields? Also, where does the script then get placed?

As for the button that needs to be made on the file, how do I attach the script to the particular button?

Thank you in advance.