Heaps of ways to do this. Try https://www.scirra.com/tutorials/449/create-a-proper-healthbar
or something simple like this:
Create sprite object for the bar with width=100. set origin image point to left.
Create variable Grow = 0
On Start Layout -> set sprite width 0
mouse click on button -> set grow = 1
Every X Sec and Grow = 1 -> add 1 to sprite width