The apps UI only works for panels.
The best you can do is to draw a button yourself and put that into your spreadsheet. Than you can add a macro to it.
Go into "Insert > Drawing...", Draw a button and add it to the spreadsheet. Than click it and click "assign Macro...", then insert the name of the function you wish to execute there. The function must be defined in a script in the spreadsheet.
Alternatively you can also draw the button somewhere else and insert it as an image.
More info: https://developers.google.com/apps-script/guides/menus
