Struggling to create an Alto Sax score with C# key signature

I've found some sheet music for Alto Saxophone that I really want to copy into Musescore.
Once done as Alto I will eventually transpose it to Tenor. But I am struggling with the Alto key signature

The sheet music in Alto is in C# and I am unable to set my score to this.

So far, I have created a score with Alto sax as the instrument.
When I add the C# key signature from the palette, oddly adds a Bb key signature.
I guess this is to do with transposing? but even if I go through all the different key signatures in the palette none of them allow me to add a C# key signature, only the enharmonic Db.

I get a Db key signature by clicking on the E Key signature, but how can I change it to the C#
(I've attached a test *.mscz file which may help in showing where I'm going wrong)

Any help appreciated.