You configure secure usernames under Administration > System Settings: SNMP ACLs. These configuration options are available under Security Names:

Displays the controls to add a security name.

Specifies a name to identify a requester allowed to issue gets and sets (v1 and v2c only). The specified requester can make changes to the view-based access-control model (VACM) security name configuration. This control doesn’t apply to SNMPv3 queries. To restrict v3 USM users from polling a particular subnet, use the RiOS Management ACL feature, located in the Administration > Security: Management ACL page. Traps for v1 and v2c are independent of the security name.

Specifies the password-like community string to control access. To enhance security, use a mix of uppercase letters, lowercase letters, and numbers. Avoid using printable 7-bit ASCII characters, except spaces, and don’t start the string with a pound sign (#) or a hyphen (-).

If you set a read-only community string (found under SNMP Server Settings), it overrides the community string you specify and grants access to the entire MIB tree from any source host. If you don’t want this, remove the read-only string.

To create multiple SNMP community strings, keep the default "public" string and add a second read-only string with a different security name. Alternatively, you can delete the default string and create two new SNMP ACLs with unique names.

Specifies the host IPv4 or IPv6 address and mask bits to which you permit access using the security name and community string.

Adds the security name.