I was going through the knowledge base and documentation. I guess An Enhancement request was opened for that matter : CSCsh20863 AnyConnect client should provide option to save username and password. Which seems that it will be not implemented because of the security implications involved with introducing such feature , so there is no way to do this.
There is other enhancement. CSCsx76993 ENH: Make Anyconnect configurable to not cache credentials
Read this from the enhancement request: Anyconnect stores username credentials within the preferences.xml file on machines. So when client is relaunched, the username is populated which could be a security risk. So, it talks about only username. Since It already save the username so lots of other customers don't want this to happen.
We could think of SSO.Unfortunately Single Sign On only works for Clientless SSL VPN, not AnyConnect SSL VPN.
~BR
Jatin Katyal
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~Jatin