To create a list item, replace the _createListItem function with the following code:
private _createListItem(): void { const url: string = this.context.pageContext.site.absoluteUrl+ "/_api/web/lists/getbytitle('My List')/items"; const itemDefinition : any = { "Title": "SPFX created item", "Info": "Info column value" }; const spHttpClientOptions: ISPHttpClientOptions = { "body": JSON.stringify(itemDefinition) }; this.context.spHttpClient.post(url, SPHttpClient.configurations.v1, spHttpClientOptions) .then((response: SPHttpClientResponse) => { if (response.status === 201) { this._operationResults.innerHTML = "Create: List Item created successfully."; this._readAllItems(); } else { this._operationResults.innerHTML = "Create: List Item creation...