How to add a ritardando sign?

There is none. Use e.g. an Expression Text (Ctrl+E) or a System Text (Ctrl+Shift+T)

See also #9383: Rubato: ritardando and accelerando

Until these "standard" texts are available in the Palette, you can create your own custom palette to provide the missing features.
See "Create a new palette" in the Handbook:
https://musescore.org/en/handbook/3/palettes
and "Save custom workspace":
https://musescore.org/en/handbook/3/workspaces#Save-workspace

I have a special palette which includes cresc. and dim. with wide-spaced lines, as well as rall., rit., accel. with lines and a few other texts without accompanying lines:

One reason why this isn't currently in the palette is there is no single tempo we could assign to this that would work for playback. So you need to customize it to your specific situation. Also usually by adding additional invisible tempo marks over the course of the passage.

Luckily, there is an excellent "Tempo Changes" plugin you can download here (see Download menu above) that automates all of this for you.

There is no way to add a real Ritardando, however if you still want to have the effect in your music, I would suggest adding a slower and slower tempo above each measure. It will slow the music down just as a Ritardando would, but it can still be done on Musescore.