That’s bizarre as you need to be “in the system” to nominate an MT.
I wasn’t even aware that there was an online process to join CPAM and I’m definitely one who much prefers online processes. When we did ours last year, it was a PDF form and the usual plethora of supporting documents.
Gosh how times have changed, when we first came in 94 all we had to do was go into the local town CPAM office with our passports and carte de séjours and see a person in the flesh. Ten mins later all signed up and they did everything and cards without photos in those days, arrived a couple of weeks later. If we needed a doctor or dentist, they would see us and give us a brown prescription or appointment form that we sent onto the CPAM for processing as nothing was on the computer in doctor’s back then, it was all forms and posting/taking them in yourselves.
That’s how it was for us when we arrived, apart from being sent (literally) in circles at first (and we had a french speaker with us as ours was pretty crap at the time). Think we were sent to 1. MMA, 2. Chambre of commerce, 3. Gites de france (go figure 
) 4. Back to CPAM and repeat before CPAM decided we went on CMU.
Yep, round and round in circles, chicken and egg scenario until someone gave way. The worst was being told by our mairie to go straight to the prefecture in Quimper which was a good two hours drive from home to apply for the CDS which the Mairie used to do in those days only to be told when we got there to go and register with the Ch de Mètiers who demanded a CDS. In the end I got cross and stamped my feet shouting at the man in the prefecture after going back again that day who then took pity and did the paperwork there and the, CDS arrived at the Mairie a few weeks later via the Bureau des Etrangers. No internet, just foot slogging to make sense of it all logically and then the real nightmares began with the business side of things and all the registrations they demanded and social charges. Glad to be out of that these days I can tell you.
Do you know what, @JohnH , we’ve had so many “impossible” resolutions of problems since we arrived that I no longer believe that “the system” exists, only miracles 
Ou, contrairement, you’ve found yourself yet another victim of the occasional demand for random, and unnecessary, details when dealing with officialdom.
CPAM processes especially whether they’re online or not might differ between Departments. I keep being told a particular process works online now each year but every time it fails. I then call up and it’s clear to me they know they’re supposed to have got their IT people to resolve what’s clearly a well known problem, even if it’s just implementing a centrally available update, and they know d*rnn well it’s supposed to work online and know I’m told that each year but they also know the Department hasn’t done the necessary fix.
This year rather than waste time trying online I just phoned up ahead and asked for a link to download the paper forms. I just said in a friendly way because you and I both know it’s supposed to work online but it doesn’t. Without demur the link to download paper forms was sent immediately. I am sure the same process does work online in a number of other Departements but suspect mine has a problen recruiting skilled IT staff.