As an ICAS member, you should have been issued with a certificate of membership when you joined ICAS. 

Your certificate will have been sent by 2nd class to the address you provided. If you have joined in the last two years and have not received your certificate, please contact members@icas.com

You can email us to request a replacement copy of your certificate. Replacement copies are free of charge and take eight to twelve weeks to produce. Certificates are proof of joining, but do not prove current membership. If you require proof of membership for a visa application or similar, you will need a Letter of Good Standing.