Add filter expression in channel member

PATCH {{_endpoint}}/services/data/v{{version}}/tooling/sobjects/PlatformEventChannelMember/:PLATFORM_EVENT_CHANNEL_MEMBER_ID

Request Body

{"FullName"=>"MyChannelMember", "Metadata"=>{"eventChannel"=>"MyChannel__chn", "filterExpression"=>"Text__c='Some value'", "selectedEntity"=>"Custom_Event__e", "urls"=>nil}}