How can I insert a custom text at the bottom left corner of the Geometry view Panel?

You can use the following Python script which creates a text annotation at the bottom left corner of all panels:

#Get current Panel boundaries
Pane=ExtAPI.UserInterface.GetPane(MechanicalPanelEnum.Graphics)
Width=Pane.ControlUnknown.width
Height=Pane.ControlUnknown.height
 
#Define the distance of the text from top left corner
identLeft = 10
identTop = Height-20
 
#Remove older annotations
g = ExtAPI.Graphics
 
for i in g.DrawContexts:
    g.RemoveContext(i)
g.Redraw
 
#Create new annotations
pt = g.CreatePixelPoint(identLeft, identTop)
annotationUser=g.Scene.Factory2D.CreateText(pt, "Test Annotation")
annotationUser.Color = 0x000000

The text's location is measured from the top left corner of the Panel. The text is displayed across all Panels, so if you need it to be removed, you can execute the following portion of the above script:

g = ExtAPI.Graphics
 
for i in g.DrawContexts:
    g.RemoveContext(i)
g.Redraw