Hi
POS functions keyboard_keydown_handler() and keyboard_handler() blocks typing of content using your keyboard with the pop-up window. To fix this ;
Create  your pos_custom.js file and override the functions from the PaymentScreenWidget. See below code
odoo.define('pos_custom', function (require) {
"use strict";
 //
 var core = require('web.core');
 var utils = require('web.utils');
 //
 var module = require('point_of_sale.models'); //Includes all Base Classes of point of sale
 var round_di = utils.round_decimals;
 var round_pr = utils.round_precision;
 //Screens
 var screens = require('point_of_sale.screens') // Require Base screens class
 //
 var QWeb = core.qweb;
 var _t = core._t;
 //
  //Extend payment screen and add more functionality screens.PaymentScreenWidget.include({
      click_paymentmethods: function(id) {
       var self = this ;
       var cashregister = null;
       for ( var i = 0; i < this.pos.cashregisters.length; i++ ) {
           if ( this.pos.cashregisters[i].journal_id[0] === id ){
               cashregister = this.pos.cashregisters[i];
               break;
           }
       }
       //debug
       console.log("Payment Register ID >> " +cashregister.journal.id) ;
       console.log("Payment Register Type >>> "+cashregister.journal.payment_mode) ;
       //
       if(cashregister.journal.payment_mode === 'mpesa'){
        var phone_no = '123456' ;
       // alert("Enter Client Phone Number >> ") ;
        self.gui.show_popup('textinput',{
           title: _t("Enter Client Phone Number") ,
           confirm: function(){
                          //get value
                          var value = this.$('#text-input').val();
                          //alert(value);
                          //console.log("Value Captured ",value);
                          if(value){
                             console.log("Perform Validation of MPESA Code") ;
                             //Add Payment Line after success validation of the code
                           self.pos.get_order().add_paymentline( cashregister );
                           self.reset_input();
                           self.render_paymentlines();
                          }else{
                            // console.log("Cannot validate Code..");
                             self.gui.show_popup('error',{
                                   'title': _t('Error: Payment Validation Error'),
                                   'body': _t('Sorry Cannot validate the payment. Try again later'),
                               });
                          }
                       },
        }) ;
       }else {
        //Add Payment Line after success validation of the code
                           self.pos.get_order().add_paymentline( cashregister );
                           self.reset_input();
                           self.render_paymentlines();
       }
   },
   init: function(parent, options) {
       var self = this;
       this._super(parent, options);
        //Overide methods
       this.keyboard_keydown_handler = function(event){
           if (event.keyCode === 8 || event.keyCode === 46) { // Backspace and Delete
              event.preventDefault();
              self.keyboard_handler(event);
           }
       };
       this.keyboard_handler = function(event){
           var key = '';
          if (event.type === "keypress") {
               if (event.keyCode === 13) { // Enter
                   self.validate_order();
               } else if ( event.keyCode === 190 || // Dot
                           event.keyCode === 110 ||  // Decimal point (numpad)
                           event.keyCode === 188 ||  // Comma
                           event.keyCode === 46 ) {  // Numpad dot
                   key = self.decimal_point;
               } else if (event.keyCode >= 48 && event.keyCode <= 57) { // Numbers
                   key = '' + (event.keyCode - 48);
               } else if (event.keyCode === 45) { // Minus
                   key = '-';
               } else if (event.keyCode === 43) { // Plus
                   key = '+';
               }else{
                return ;
               }
           } else { // keyup/keydown
               if (event.keyCode === 46) { // Delete
                   key = 'CLEAR';
               } else if (event.keyCode === 8) { // Backspace
                   key = 'BACKSPACE';
               }
           }
           self.payment_input(key);
          // event.preventDefault();
          /* if (event.type === "keypress") {
             return ;
            }*/
       };
       //End method override
   } ,
}) ;
});
Thanks..