It's really hard to find something for a windowbox that can take the wind. I have generally resorted to wax begonias in my windowboxes most years, although this year I'm trying some hummingbird plants (salvias - eg., S. coccinea 'Lady in Red' and 'Hummingbird Red'). The begonias have a dense habit and thick leaves that seems to help them remain stiff when it gets windy. I had geraniums (pelargoniums) for a number of years, although not in a windowbox. They are pretty stocky and should do well. The main drawback is that when it gets too hot, they stop blooming. They are actually cool-weather plants - ie., much of Europe except the furthest southern parts, don't have the type of sustained heat and humidity that many U.S. locales (both north and south) have in summer. Although the past couple summers, many Europeans have been treated to a few unusual (for them) heatwaves which have sadly caused many deaths as most have no AC. Another possibility that I have seen work in hot windy is the vinca flower ("Madagascar vinca"). The plant is remiscent of impatiens but the stems and leaves are very different and these can take full hot windy blazing sun and come in a myriad of colors. Another choice might be the marigold. They have always been standard full-sun plants, come in different colors including the yellows and oranges, and come in different sizes. About 7 years ago when one of my sisters was still in an apartment with a south-facing balcony, she had marigolds and dwarf sunflowers in her windowboxes. As something totally different - you could consider some sedum, whether the various creeping ones (eg., stonecrop), or the more upright ones (if pruned to keep compact like "Autumn Joy").. They are later bloomers but with the fleshy leaves, they might be less likely to dessicate in the wind.

You will need to swap out the soil in the pots in spring as the salt makes it unusable and dispose of it somewhere that you don’t want plants to grow. For future reference, check out how the Detroit area landscape designer does her winter pots using large sheets of foam cut to size to sit inside the container. The greens, branches and ornamental picks are placed in the foam in a slightly warmer workspace, and then the finished pieces placed into the pots. https://deborahsilver.com/blog/the-winter-pots-a-visual-tutorial/

Geraniums (actually pelargoniums, see the FAQs if you want the expalanation of the difference) need to be brought in before hard freeze. They can take light freeze if they are covered at night with an old blanket, or a couple of bedsheets. They can be grown indoors as houseplants, and will reward you with beautiful blooms to enjoy all winter, as they would in their native Africa. I grow mine in a lightly heated sunroom all winter, because it's got nice light. They'd like a bit more warmth to put out more blooms, but they survive winter nicely in there. If you are blessed with bright southern windows in your house, they'll really do well.

It is one of those geraniums which extend outwards from new leaf nodes - like psilostemon or wallichianum, rather than manitaining a clump form such as sanguineum...but given a chance, it will scramble upwards through surrounding taller plants, popping up, seemingly at random between close growing perennials and shrubs. Not really suited as a specimen plant - but looking good in a 'wild' or informal garden.