ui_mainwindow.h: No such file or directory: Compile Qt with MSYS2

I want to compile a Qt-Project without an IDE like QtCreator and I use MSYS2.

I installed Qt5 with

pacman -S mingw-w64-x86_64-qt5-base

I could then already create an application with

main.cpp

#include <QApplication>
#include <QPushButton>

int main(int argc, char **argv)
{
 QApplication app (argc, argv);

 QPushButton button ("Hello world !");
 button.show();

 return app.exec();
}

and myproject.pro

TEMPLATE = app
TARGET = name_of_the_app

QT = core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

SOURCES +=  main.cpp

and the commands

qmake
mingw32-make
./release/name_of_the_app.exe

However, now someone sent me a QtApplication a bit more complex and it contains mainwindow.ui and MainWindow.h

The start of those files looks like

mainwindow.h

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>1014</width>
    <height>643</height>
   </rect>

MainWindow.h

#pragma once

#include <QMainWindow>
#include <QStandardItemModel>
#include <QDate>
#include "Patient.h"
#include "MedikamentenplanAnzeigeModel.h"
#include "BehandlungenAnzeigeModel.h"

namespace Ui {
    class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

The guy, who sent me the project hasn't sent a .pro file. I want to write that myself. He has sent me a VisualStudio .sln and .vsprojx files

How do I need to adapt my project file. I already added

HEADERS += MainWindow.h

And also

QT += widgets

but still mingw32-make can't find ui_mainwindow.h. Of course, I am completely new to this Qt.

You need add

FORMS=mainwindow.ui

to the myproject.pro