How to jump in a platformer?

For jumping in a platformer, you need gravity. And for gravity, you need a vertical speed variable.

The gravity would be a constant value, which is added to the vertical speed:

vsp += grav; // Add grav to vertical speed

y += vsp; // Add vertical speed to y, to move the instance

When you need to jump, you can simply set vsp to a negative value:

if (keyboard_check_pressed(vk_space) && grounded) {
    vsp = -10;

grounded would store if the player is standing on the ground.