There is no command for dialog boxes. Rather, they are made by outputting more lines and text to the screen. It's also good to have drop shadows behind the dialog boxes. And yes, this is all possible in BASIC.
When creating a dialog box, it is best to clear the portion of the screen where you will be putting your dialog box. Do this with a forloop and the inverse line command. Let's say we want to make a dialog box that takes up most of the screen, that is, from -8,8 to 8,-8. Here is the code for clearing the screen:
:For(I,8,-8,0.2)
:Line(-8,I,8,I,0)
:End
Next, you'll want to draw your outline of the box and the drop shadow. This is done with 6 lines. In the example above, the outline would be like this:
:Line(-8,8,8,8)
:Line(8,8,8,-8)
:Line(8,-8,-8,-8)
:Line(-8,-8,-8,8)
The drop shadow is done by two lines 0.3 from the left and bottom of the dialog box. For example:
:Line(8.3,7.7,8.3,-8.3)
:Line(-7.7,-8.3,8.3,-8.3)
Now that the dialog box is set up, you can begin to load content into it. Dialog boxes usually have a title, content text, and one or two buttons. The text is done just by the Text( command, and the button is done by more lines and text. So, using a dialog box requires a lot of code, but it's worth it for the graphics and the interface with your user.