Check out Krita scripting school:
https://scripting.krita.org/lessons/introduction
Also check out:
This should help you get started once you are done reading:
from krita import *
class TestDialog(QDialog):
def __init__(self, parent=None):
super().__init__()
self.setWindowFlags(Qt.Popup)
self.move(QCursor.pos())
dlg = TestDialog()
dlg.exec_()
To finish this, just have your bar connect to the one in the QDoubleSpinBox in the layout.