FastAPI POST 422 Unprocessable Entity error

This is my FastAPI script, I am doing basic lesson to make the POST API as simple as possible.

class Item(BaseModel):
    test_param:str

@app.post('/myapi')
async def myapi(item:Item):
    print(item)
    return {"myapi":"OK"}

Then my script is like this:

var formData = new FormData();
formData.append('test_param',"1");
    axios.post("/myapi",formData
    ,{headers: {'Content-Type': 'application/form-data'}}).then(res=>{
console.log(res);
});

When calling API, it shows the error 422 Unprocessable Entity.

I guess there is some discrepancy in API and script, however where should I fix?