To assign the first row of the "ID" column in the "DT" datatable to a String variable, the expression that should be added to the Value field of the Assign activity is:
DT.Rows(0).Item("ID")
This expression accesses the value of the "ID" column in the first row of the "DT" datatable using the Rows and Item properties. The Rows property returns a collection of DataRow objects that represent the rows in the datatable. The Item property returns or sets the value of the specified column in the DataRow object1. The expression uses the index 0 to refer to the first row in the Rows collection, and the column name "ID" to refer to the specific column in the Item property. The expression returns the value of the "ID" column in the first row as an Object type, which can be converted to a String type using the ToString method2. For example, if the "DT" datatable has the following values:
ID
Name
1
John
2
Mary
3
Bob
Then the expression DT.Rows(0).Item("ID") will return 1 as the value of the "ID" column in the first row.
References: DataTable.Rows Property and DataRow.Item Property from UiPath documentation.