How do I fix my shower form leaking when tap is on running water in tub

We will move you to plumbing so you get better answers. Do you mean your shower is partially running when you just want the tub to fill? If so your diverter is partially stuck open. What brand mixing valve do you have?

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Do you have diverter on top of the spout or 3 handle faucet ?

do you have diverter on top of the spout or 3 handle faucet ?
Milo asked asked a pertinent question. My guess is that you have a spout diverter,( see image). If that's the case then the first thing I would check is the house pressure. The only thing preventing the shower from leaking when you fill the tub is 27 inches of back head pressure in the shower raiser. If the house pressure's excessive then It will over ride the back pressure and send water up the raiser. Do you have a PRV( Pressure Reducing Valve) installed where the water service enters the house? If I guessed wrong then click on back with the brand name of the valve, Regards, tom