You want the CommandLine$() function.
This will return the path+filename of the file opened.
Example
BlitzPlus program
Saved as TEST.EXE
File assocated with TEST.EXE
myfile.tst
Saved in C:\TEMP
Double-Clicking myfile.tst would cause the example TEST.EXE to produce:
C:\TEMP\myfile.tst
This will return the path+filename of the file opened.
Example
BlitzPlus program
Notify CommandLine$() End
Saved as TEST.EXE
File assocated with TEST.EXE
myfile.tst
Saved in C:\TEMP
Double-Clicking myfile.tst would cause the example TEST.EXE to produce:
C:\TEMP\myfile.tst