Now, we need to add an entry in /etc/shadow for this particular user. To do this, type vipw -s. You'll be greeted with your favorite editor, which now contains the /etc/shadow file. Now, go ahead and copy the line of an existing user account (i.e. has a password and is longer than the standard system account entries):

drobbins:$1$1234567890123456789012345678901:11664:0:-1:-1:-1:-1:0

Now, change the username on the copied line to the name of your new user, and ensure that all fields (particularly the password aging ones) are set to your liking:

testuser:$1$1234567890123456789012345678901:11664:0:-1:-1:-1:-1:0

Now, save and exit.