.netstandard is cross platform, so MSMQ which depends on windows, can not implemented in .netstandard (which is a base api).
MSMQ, like WCF did not make the cut to be ported to net core.
your practical solutions based on requirements are:
1) if you only need queueing support, create a rest api website in 4.8 that supports queuing. you .net 6 apps can call this site.
2) if you need a .net 6 app to receive queue message, use a rest api, and have a 4.8 MSMQ app call the rest api.
long term you need to pick a new messaging standard, as MSMQ is fairly dead.