It shouldn't be more than a few lines of code. Create a keyboard layout on screen with the letters/numbers you want as buttons and a text field/panel that will display the results. Then create a PS action for each button that calls Keyboard("A"), Keyboard("9"), etc.

In the activity level Keyboard() function, you can either buffer the character to a string variable or append it directly to the panel's label. Add a "DEL" key and put code in to do a substr or slice function to truncate the string if you need to make corrections.

Finish off with a "Go" or "Enter" button that will then call your weather feed code with the completed string.