How to pass string (not file) to openssl?

Use this:

user@host:~$ echo "my string to encrypt" | openssl aes-256-cbc -e -a -K 00000000000000000000000000000000 -iv 00000000000000000000000000000000
a7svR6j/uAz4kY9jvWbJaUR/d5QdH5ua/vztLN7u/FE=
user@host:~$ echo "a7svR6j/uAz4kY9jvWbJaUR/d5QdH5ua/vztLN7u/FE=" | openssl aes-256-cbc -d -a -K 00000000000000000000000000000000 -iv 00000000000000000000000000000000
my string to encrypt

Or you could use command substitution:

user@host:~$ openssl aes-256-cbc -a -K 00000000000000000000000000000000 -iv \
00000000000000000000000000000000 -in <(echo "my string to encrypt") -out encrypted.txt

Enter multiline input, use ctrl+d to finish. e='e' will clear the env var, for privacy.

e=$(cat); echo "$e" | openssl enc -aes-256-cbc -e -a; e='e'