Method to generate vector using camera location as origin from mouse click

i am familiar with getting camera position in world.(x,y,z) but getting (i,j,k) vector based on mouse click evades me. trying to be able to click on an arbitrary point on a sphere defined with a center and a radius. the math to find out if a vector intersects exists, as does the formula for finding the vector/sphere intersect point(s). (point closest to camera can be determined by selecting the point with least distance to the camera using the 3d distance formula)

problem is that i can’t figure out how to get the vector from a mouse click to fill in the blanks of the equations to complete my task. any help would be greatly appreciated.

ended up using cell type to get the x,y,z cords of a ray hitting a surface. did the needful