Not sure thats possible. But there are some workarounds like using a keyfile on a usb or a yubikey which can output a static password on longpress. Or some other hack-around.
Some relevant search results, the first one seems especially promising:
https://forum.garudalinux.org/t/unlock-luks-with-a-touch-screen-on-screen-keyboard/24208
https://www.reddit.com/r/Fedora/comments/a22ehu/fedora_29_luks_passwd_on_boot_with_touchscreen/
https://www.reddit.com/r/SurfaceLinux/comments/9g5ooa/onscreenkeyboard_for_lukslvm_encryption/