How do you link an exit button to debrief page at the end of the study?

If you have a URL for your debrief page, you could try specifying onclick in the code for your button, like this:

https://www.qualtrics.com/';"
 title="Exit Survey" type="button" value="Withdraw" />


Alternatively, you could add a question to the end of each page where participants can click to withdraw from the survey. Make it a multi-select question (and make the option to withdraw exclusive if you have more than one option available) - that way they should be able to deselect the yes/withdraw option if they accidentally click it. If they select "withdraw" in that question, then skip logic can take them to the end of the block. Branching logic in your survey flow could then take them to the end of the survey with a customized survey end/message - whatever your debrief is. 

Here is another Qualtrics community post about this topic. And here is a post by Tyler Burleigh you might find helpful.

                                
            
                        

You could try uploading the debrief page text to your files library in Qualtrics. Upload a new file by going to Library --> Files Library --> Upload a new file.
Then you could get a URL for the file. You can copy/paste the URL that you get when you click to view the file. It should open the file as a PDF in a new tab in your browser). The URL would look something like: https://yourcompany.az1.qualtrics.com/WRQualtricsControlPanel/File.php?F=F_somethinghere