Nicknames can only be bestowed. Giving yourself one never seems to work. I know. I tried to get my granddaughters to use 'Gramp Poobah' but it never took. Their grandmother wanted to be called 'nanna' so, of course, she is now 'gramma nanna'.
Well, it seemed to have worked for "von Marwitz" and "the Oberst". You have to give people something that they can picture in their mind, that they like or at least recognize, and then "play that role" - tenaciously.
The nickname was originally chosen for a tank rumble in ye olde days in which there were limited commuications via the tanks' radios. Most players were Americans and these tend to like harsh sounding German names - if there is a "von" in it, all the better. At least, they include such things in every WW2 movie in creation originating from the US. Apparently, "it sells"...
Then give them pictures to form in their minds: Oberst Stransky from the movie "Steiner - Das Eiserne Kreuz" anyone? Playing with some well established stereotypes works as well, the spit-polish Prussian officer for example.
Slap a 6+1 on the forum avatar along with the rest, then give'em some chatter to play along these lines. Rinse & repeat for a decade or so. Don't forget to lose big time at some tournaments lamenting the injustice of the material superiority of the Allies for good measure...
There you go.
von Marwitz