Async config before Angular bootstrapModule

Where is the best place to call load()

You can call load function in the App module. Angular provides a way to achieve it. You can use APP_INITIALIZER. The provided functions are injected at application startup and executed during app initialisation. If any of these functions returns a Promise or an Observable, initialisation does not complete until the Promise is resolved or the Observable is completed.

function initializeApp(): Promise<any> {
  return new Promise((resolve, reject) => {
    // Do some asynchronous stuff
    resolve();
  });
}

@NgModule({
 imports: [BrowserModule],
 declarations: [AppComponent],
 bootstrap: [AppComponent],
 providers: [{
   provide: APP_INITIALIZER,
   useFactory: () => initializeApp,
   multi: true
  }]
 })
export class AppModule {}

How would I take that resolved MyConfig from Promise and inject throughout my Angular app?

Now, this can be done in number of ways, but I would suggest to create a service at the root level to keep the data in one place then you can pass it to the store or save on localStorage whatever fits your case.

Config Service:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import 'rxjs/add/operator/toPromise';
import { map } from 'rxjs/operator/map';

@Injectable()
export class AppConfigService {

  private configData: any;

  constructor(private http: HttpClient) {}

  load() :Promise<any>  {
      const promise = this.http.get('URL').toPromise()
                      .then(data => {
                          return data;
                       });
      return promise;
  }
  // once you get the data set it;
  setConfigData (data) {
       this.configData = data;
  }

  getConfigData (data) {
      return this.configData;
  }
}

In you App.module.ts :

import { NgModule, APP_INITIALIZER } from '@angular/core';
import { AppConfigService } from './services/app-config.service';

export function appInit(appConfigService: AppConfigService) {
  return new Promise((resolve, reject) => {
    // Do some asynchronous stuff
    this.appConfigService.load().then(data=> {
             this.appConfigService.setConfigData(data);
             resolve();
    })
    
  });
}

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [

  ],
  providers: [AppConfigService,
    {
      provide: APP_INITIALIZER,
      useFactory: appInit,
      multi: true,
      deps: [AppConfigService]
    }],
  bootstrap: [AppComponent]
})
export class AppModule { }

you might be looking for APP_INITIALIZER