I need help writing a script for a Dialogue Pop up

Hello I want to make button with a script that displays a custom Dialogue Pop Up window. I've searched for guides for how to do it like this, or this but nothing has been helpful for what I need.

Here is tha screnario:

We have three text fields: "Text 1", "Text 2" and "Text A".

There is a button with a script. When you press it checks if there if Text 1 and Text 2 are blank. If there is any text in both of them, a dialogue Pop-Up appears with two buttons. The text in the buttons is thext input in Text 1 and Text 2 respectively.

When you click on one of the buttons, it swaps the text input of the field Text A with the input of Text 1 or Text 2 depending on which button was pressed. i.e. if I press button 1 (which diplays the text from Text 1) the string from Text A is moved to Text 1 and the string from Text 1 is moved to Text A.