Problem with key signatures on musescore

If I am writing music for alto saxophone that is in the key of E major concert, I could either write the piece in C-sharp major with seven sharps in the key signature, or I could write it in the key of D-flat major with five flats in the key signature. However, musescore only gives us one option, and that is to write it in the key of C-sharp major with seven sharps. The reason why is because E-major concert transposed for an E-flat instrument is C-sharp major and not D-flat major. D-flat major for an E-flat instrument is actually the key of F-flat major in concert pitch which is a theoretical key, and musescore does not allow theoretical keys for some reason. I would very much prefer to use D-flat major instead of C-sharp major, but I cannot. I tried using the "Create Key Signature" option in the master palette to create that key, but it was obsolete. It did not function like an actual key signature; it was just for show. Does anyone know any way to fix this? I would really appreciate it!