People cross their fingers when they hope for a favorable outcome or wish for good luck. Wikipedia has
To cross one's fingers is a hand gesture commonly used to wish for luck. Occasionally it is interpreted as an attempt to implore God for protection. The gesture is referred to by the common expressions "cross your fingers", "keep your fingers crossed", or just "fingers crossed".
Some people, mostly children, also use the gesture to excuse their telling of a white lie. By extension, a similar belief is that crossing one's fingers invalidates a promise being made.
There are probably few different ways to describe the version with the hands. I think the most common way to say this (at least in AmE) might be folding your hands, though I might be forgetting one.
fold one's hands
to bring one's hands together, palm to palm, with the fingers interlocking; to grasp one's hands together, palm to palm, perpendicular to one another. Please fold your hands and put them on the table while the teacher reads you a story. Please fold your hands and be quiet.
You can also say clasp your hands, interlace your fingers, interlock your fingers. Clasp your hands could also mean cupping and grasping your hands, without interlocking the fingers. Also, hand clasping seems to also be scientific jargon.