Hello Tonka,
You can find definition of this button in the related control file:
\CMS\CMSModules\MessageBoards\Controls\Messages\MessageEdit.ascx
around line 112:
<cms:LocalizedButton ID="btnOk" runat="server" ButtonStyle="Primary" OnClick="btnOk_Click"
EnableViewState="false" ResourceString="general.add" />
So as you can see, there is the resource string: general.add used. You can create a custom Resource string and change it here.
Feel free to check the Documentation to learn more about
Resource strings and how to work with them.
Regards, Martin