How automatically rename video file

Hello
In MKVToolNix, when I want to mix 2 files - one video and one subtitle file (srt), I want the output filename to be different from the original video file.
For example, input video file:
This.is.nature.720p.mp4

And the output should be:
This.is.nature.720p (1).mp4

If I remember correctly, this could be done in the destination file name section, but unfortunately I don’t remember how.

Can you guide me?
I want these settings to be permanently active and enabled by default every time I run the program.

MKVToolNix can only create Matroska files. Therefore it cannot & will not use .mp4 as the extension for the destination file. What happens is that it automatically sets .mkv as the extension & then looks for existing files with the same name (including the extension .mkv). Only if a file with the same name & .mkv extension is found will MKVToolNix insert trailing numbers to make them unique.