There are detailed instructions to use the melt command line here:
https://www.mltframework.org/docs/melt/
Shotcut does NOT use the melt command line parameters to modify the export parameters. Rather, the MLT XML structure has a feature to embed the export parameters into an XML file that is passed to melt. The export XML looks exactly like the project file, but it has an additional <consumer> tag.
As a first step, export a project from Shotcut. In the Jobs panel, right click on the completed job and choose “View XML”. That will open a window that shows the contents of the export file. Compare the export file to the project file and you will see the subtle difference.
The command to export from the export file is simply: melt exportfile.mlt (of course, the name of the file is not actually “exportfile”).