Liquid zero start of string fill-ins.

zeros.liquid

{% assign user_code_str = event.user.user_code | downcase %}
{% capture user_code_length %}{{user_code_str | size}}{% endcapture %}
{% capture zeros_missing %}{{'9' | minus: user_code_length}}{% endcapture %}
{% capture nine_digit_user_code %}{% for zero in zeros_missing %}0{% endfor %}{{event.user.user_code}}{% endcapture %}