I want an extra window at my leasehold apartment. What do I need to do.?

I presume that you are in the UK and that you have a conforming leasehold. You would get more accurate answers if you had been bothered to include this information.

For an additional window you are going to need two things; planning permission and freeholder permission. The council will need plans and elevations. The freeholder is likely to also need an engineering survey to make sure that the new window does not compromise the structure of the building.

Start by checking to see if you are in a preservation area. If you are, you will not be able to alter the façade but you might be able to put something at the back of the building. Then check with your freeholder. Find out if he would be willing to work with you. No sense in laying out money on plans and surveys if your freeholder will not allow you to do the work.

You could ask the owner for permission to have one installed, but I doubt they would authorize it.

ask the landlord, but the odds are 0