From The Document Foundation Wiki
I have a column for given names and one for surnames, I want a third one with given_name.surname.
You need to "add" the cells together, but with "&" and not "+", because this is text.
You can also add a space or a full stop between the two character strings, again by using "&".
- cell B2 is the cell that contains the given name;
- cell A2 contains the surname;
- &"."& adds the full stop between them.
It is also possible to use this function: