Google's Material Design Framework has popularized a design pattern for a sticky button on the bottom of the screen known as the FAB, or Floating Action Button.
Google has specific guidelines for the appearance and behaviour of this button, for example:
The floating action button should be placed 16dp minimum from the edge on mobile and 24dp minimum on tablet/desktop.
The floating action button changes color on focus and lifts upon being selected.
A floating action button represents the primary action in an application.
Only one floating action button is recommended per screen to increase its prominence. It should represent only the most common
Make floating action buttons positive actions like Create, Favorite, Share, Navigate, and Explore
However, since you are not using the Material Design framework specifically, you do not have to following all of these guidelines exactly.
Instead, concentrate more on the user's expectations and familiarity with the pattern -- if users see a FAB-type button, would they understand how it works? What are the kinds of behaviors they would expect?
For example, other sites such as https://www.intercom.com/ do not use the Material Design framework, but still use a FAB-type button.
Finally, one issue you may encounter is that the FAB button typically does not use text labels. So you may struggle to find an icon that suggests 'Apply'. Or you may have to break convention slightly here and employ a text label.