crossSiteCreateListItem.ts

private async createListItem() {
    try {
      const spOpts: ISPHttpClientOptions = {
        body: `{ Title: 'My New Item' }`
      };

      const response = await this.context.spHttpClient.post(`https://tenant.sharepoint.com/sites/anothersitecollection/_api/web/lists/GetByTitle('test')/items`, SPHttpClient.configurations.v1, spOpts);

      if (!response.ok) {
        const responseText = await response.text();
        throw new Error(responseText);
      };

      const responseJSON = await response.json();

      console.log(responseJSON);
    } catch (error) {
      console.log(error);
    }

  }