The minimal investment amount depends on the instrument price and position volume.
For example, when buying 1 share of Tesla, which costs $863, you'll need $863 to open the investment.
Because accounts at the company are not expressed in dollars, one would need about €742 to buy 1 share of Tesla. Your account's currency will be automatically converted to the stock's currency, and the purchase will be made at the corresponding exchange rate.
One share in a presented company is the minimum purchase volume.