On mobile, you really just spell the word in hiragana and your phone should change the word to katakana if appropriate. For example, if I wanted to spell アメリカ I would type it in hiragana and either click on the suggested katakana spelling when it pops or press the space bar or 半角／全角 button.
I'm not entirely sure if your keyboard has this, but there should be other settings for keyboards, right? If not, then it's pretty much what the previous suggestion says
You should be able to turn the characters into katakana while writing hiragana. When you're done writing, the space key (above enter) should turn into katakana what you just wrote. If it wants to turn in katakana more than what you want to, press enter (it won't make a new line inmediately) and then write what you want to turn into katakana.
I think it depends on your phone, but if I spell something in hiragana, my phone automatically suggests the katakana spelling on top of the word as well as other suggested words, but I just click on the katakana spelling I want to use.
On Simeji keyboard there's a key for it, that appears when typing a word in Japanese. But the Renshuu app accepts hiragana for katakana. So you don't need it to use it.
