The trading platforms will not allow you to take trades that will bring your margin utilization above 100%.
In the trade ticket, you will have a margin preview available, which gives you an indication of how much Initial margin available you have.
If the Initial margin impact is higher than the Initial margin available, your trade will be rejected.
In such a case, you can reduce the size of your position to match the Initial margin available and try again.
Please, be aware, that the initial margin is shown without taking into account existing open orders. Existing open orders are only calculated during new order placement.