How to use api to find out the part name during creating drawing?

The basic version of that would be:

drawingPath = Left$(swModel.GetPathName, (Len(swModel.GetPathName) - 6)) & "SLDDRW"

I had to look up the function of the dollar sign in VBA, it's to make sure Left returns a string. You can add other necessary checks accordingly.

You can use this statement to get the document name without its path and extension

sModelName = Left(swModelDoc.GetTitle, InStrRev(swModelDoc.GetTitle, "-") - 2)