To disable ProjectWise login screen to appear when you launch OBM please add the following files to the "OpenBridgeModeler.cfg" file. The location of the CFG file for the latest released version (10.10.20.91) is: "C:\Program Files\Bentley\OpenBridge Modeler CE 10.10.20\OpenBridgeModeler\config\appl". Add the "#" sign if you want to get the PW login dialog or remove it to not get the PW login dialog when you open OBM:
#--------------------------------------------------------------------------------------
# Disable ProjectWise
#--------------------------------------------------------------------------------------
# To disable ProjectWise, uncomment this variable
PW_DISABLE_INTEGRATION=1
#--------------------------------------------------------------------------------------
# Disable ProjectWise
#--------------------------------------------------------------------------------------
# To disable ProjectWise, uncomment this variable
PW_DISABLE_INTEGRATION=1