Simplest way is to create extractor to extract timestamp (e.g usgin regex) from full_message and add Date converter to parse date:
Source field: full_message
Regular expression: <[0-9]+>(.*): MX480-CGNAT
Condition: Only attempt extraction if field contains string: MX480-CGNAT
Store as field: timestamp
Extraction strategy: Copy
Add converter: Date
Format string: yyyy-MM-dd HH:mm:ss
Timezone: select timezone