How to file itr for income from stock market for govt emp.

Hi, I am a Govt employee but I also invest in share market and I have short term capital gain from investment in stock market.

1) Which form I should use for filing ITR?  ( When I login to efiling it only shows ITR1 and ITR4 but I can't find where to show income from stock market in ITR 1 )

2) What is the last date to pay Income Tax on income from share market? ( Income tax on salary has already been deducted by the employer and I have also asked them to deduct some additional tax as I had this other income but I still have some tax outstanding when I include the income from share market. Can I pay it at the time of filing ITR)