I've made some progress!

I've replaced:

/usr/bin/md5sum --string\
[download]

With

/usr/bin/md5sum <<<\
[download]

and got:

Password Successfully Added.

Password: 123456

MD5 Check Sum: F447B20A7FCBF53A5D5BE013EA0B15AF

but this does not look like an MD5 Hash Code, and when I check an online generator... well, it's not the same.

Seeing that I don't really know what I'm doing (I'm googling), it's hit or miss. Maybe it's supposed to look like this. I'm going to enter the code on the client side of the API and see if it works. If it does, I'm done!