Hi there, there are 2 ways that I know of to do this, both of which involve putting a button in the Header of the survey and then making the Header sticky so that the button is always visible to the respondent. First, create a button.
1) If your survey is using Anonymous Links, you can have a button that takes the respondent to a website outside of their survey session. Just make sure that "Allow respondents to finish later" is not toggled in the Survey Options. Try putting the below in the Header of your survey:


2) If your survey is not using Anonymous links, then you'll want the button to facilitate closing the survey session instead of leaving it. This can be done by adding a button within the Header that will set an Embedded Data field value to "1" or "true" or something similar and then advance the page. Then, the rest of the survey can use Display/Skip logic so that respondents only see the rest of the survey if the Embedded Data field is not equal to whatever the "End Survey" button would set it to. ahmedA describes that method in the below thread:

https://community.qualtrics.com/XMcommunity/discussion/13744/add-exit-survey-option-throughout-and-have-conditional-flow-logic-to-exit-message

Finally, by adapting the MattyB's code in the below thread, we can make the Header sticky. Put the below code in the CSS of your survey's Look & Feel:
#SurveyEngineBody {
  /* Don't want to cover the top of your survey with the button div */
  padding-top: 50px;
}
#HeaderContainer {
  top: 0;
  position: fixed;
  width: 100%;
  left: 0;
  right: 0;
  background: #fff;
  background-color: #fff;
  border-bottom: 1px solid #ddd;
  margin-top: 0;
  z-index: 999;
}
https://community.qualtrics.com/XMcommunity/discussion/1431/floating-submit-button