Thanks for your update.
QUERY: How to preselect with “showCheckbox” option
From your query we understand that you need to use “showCheckbox” property, in order to select particular record’s checkbox at initial rendering itself.
This can be achieved by selecting those checkboxes using “selectCheckboxes” method in “dataBound” event.
Please refer the below help documentation,
In dataBound event, you need to call the selectCheckboxes method with the rowIndices (rowindices of records which you need to preselect) as parameter.
Please refer the below sample,
If you have further queries, please get back to us.