Get access or point to SAP UI5 control

You are making a small mistake over here. Method addContent is a method available for UI5 controls, not over normal DOM elements.

Accessing SAP UI5 Controls

If you want to gain a reference to a UI5 control and add content to it, you can use the byId method to get access to the control by its ID. This method is available in views and controllers.

Example

Here's how to get a reference to a toolbar control and add a button to it ?

this.getView().byId("<id of the toolbar>").addContent(new sap.m.Button({
    text: "My Button",
    press: function() {
        // Button press handler
        console.log("Button clicked");
    }
}));

Alternative Methods to Access Controls

You can also use other methods to access UI5 controls ?

Using sap.ui.getCore()

For controls with global IDs ?

var oControl = sap.ui.getCore().byId("globalControlId");

Using Controller Reference

In controllers, you can directly use this.byId() ?

var oToolbar = this.byId("toolbarId");
oToolbar.addContent(new sap.m.Button({
    text: "Dynamic Button"
}));

Conclusion

Always use the appropriate UI5 methods like byId() to access controls rather than DOM manipulation. This ensures proper integration with the UI5 framework and maintains the MVC architecture.