@LanX, thanks. I figured I wasn't doing something correctly. In all my years I've never ever used a system like this.

@BrowserUk, nope. "--text" did not work. It yielded the following:

[Sat Feb 11 18:18:06.310530 2017] [cgi:error] [pid 32673] [client 97.9
+3.113.253:50798] AH01215: Try `/usr/bin/md5sum --help' for more infor
+mation.: /home/beee001/public_html/cgi-bin/eposturl.cgi

[Sat Feb 11 18:18:06.310432 2017] [cgi:error] [pid 32673]
[client 97.93.113.253:50798] AH01215: /usr/bin/md5sum: option '--text'
+ doesn't allow an argument: /home/beee001/public_html/cgi-bin/epostur
+l.cgi
[download]

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!

I just processed a transaction and it works!

any ideas why the code doesn't look like an MD5 hash code?