要在Angular 7中在请求头中发送PAYLOAD，可以使用HttpInterceptor来拦截请求并在请求头中添加所需的参数。以下是一个示例代码：

首先，创建一个名为AuthInterceptor的HttpInterceptor类：

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class AuthInterceptor implements HttpInterceptor {

  intercept(request: HttpRequest, next: HttpHandler): Observable> {
    // 在请求头中添加payload参数
    const modifiedRequest = request.clone({
      setHeaders: {
        'PAYLOAD': 'your_payload_value'
      }
    });

    return next.handle(modifiedRequest);
  }
}

然后，在app.module.ts文件中将AuthInterceptor添加到providers数组中：

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';
import { AuthInterceptor } from './auth.interceptor';

@NgModule({
  declarations: [
    // ...
  ],
  imports: [
    BrowserModule,
    HttpClientModule
  ],
  providers: [
    {
      provide: HTTP_INTERCEPTORS,
      useClass: AuthInterceptor,
      multi: true
    }
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

现在，当你发起HTTP请求时，AuthInterceptor将拦截请求并在请求头中添加'PAYLOAD'参数。你可以根据需要修改'PAYLOAD'的值。