I want to create custom button to simulate "Next" button of keyboard
I imagine you have a sequence of Text Boxes for input, and you want to jump to the next text box after validating the input to the current text box that has focus.
You can init a global list with component blocks for your text boxes, in the order that you want the next button to follow.
There are textbox blocks to control focus, which is where the input goes.
You could add an extra global variable to hold the component block for the current text box, or instead have a global index variable (1 to length of list of your textbox list) to make it easier to know where to look for the next textbox, and to know when you have reached the last textbox and should begin work on saving your input.
You will need generic textbox blocks for this, most likely.
This extension might help App Inventor Extensions: Textbox | Pura Vida Apps
I need this "Next" button for web page, I'm using CustomWebView