Set static prefix

PrefixInputFilter

class PrefixInputFilter(private val prefix: String) : InputFilter {

    override fun filter(source: CharSequence, start: Int, end: Int, dest: Spanned?, dstart: Int, dend: Int): CharSequence? {
        val newStart = max(prefix.length, dstart)
        val newEnd = max(prefix.length, dend)
        return if (newStart != dstart || newEnd != dend) {
            val builder = SpannableStringBuilder(dest)
            builder.replace(newStart, newEnd, source)
            if (source is Spanned) {
                TextUtils.copySpansFrom(source, 0, source.length, null, builder, newStart)
            }
            Selection.setSelection(builder, newStart + source.length)
            builder
        } else {
            null
        }
    }
}

EditText.filters = arrayOf(PrefixInputFilter(prefix))