Do you want to add the thousand separator in a string, or do you want to display it with a thousand separator on a page?
If you just want to display it on a page, use a decimal attribute. Then open the properties of the value you are displaying and select ‘Group digits’ as shown below.
If you want to add the decimal separator in the actual string value you do need to write logic in a microflow that processes your data. You can use the formatDecimal function for this, as can be found here: https://docs.mendix.com/refguide/parse-and-format-decimal-function-calls
If you parse this value:
the output will be:
Please note that it also depends on your language settings.
See the documentation here: https://docs.mendix.com/refguide/columns-document-template
1.4 Group Digits (Only for Numeric Attributes)
For ease of reading, numbers with many digits before the decimal separator may be divided into groups using a delimiter. This property defines whether the end-user will see these groups, or not.