Apple - Send Enter key using the mouse

Something like BetterTouchTool can be used for this purpose. You can configure gestures and actions that trigger keyboard shortcuts. You could trigger something that simulated the enter key being pressed using this tool.

1. I think you are asking about virtual keyboard on Mac

You can show it by doing this (screenshots are by the link provided above):

• Open System Preferences and go to “Keyboard”, and look under the “Keyboard” tab
• Check the box next to “Show Keyboard & Character Viewers in menu bar”
• Pull down the newly visible Keyboard menu and choose “Show Keyboard Viewer”
• Place the keyboard on screen in the desired location, and resize the newly visible keyboard as necessary by dragging the corners

2. Second option to use third party application for configuring shortcuts/mouse actions and guestures, alike BetterTouchTool. There could be other Applications as well.