When adding a filter to a report using a record's Salesforce ID, users may notice that entering a 15-character ID will automatically convert it to the full 18-character version once the filter is applied.
Salesforce report filters use the 18-character record ID, but the report UI displays only the 15-character ID in the column. If you need to display the 18-character Record ID in your reports, you can achieve this by creating a Formula field.
To convert 15 - character ID to 18 - character ID, please refer the below article:
https://help.salesforce.com/s/articleView?id=000385066&type=1
005131264

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