second parameter is default value, here is example code:
data = [{"Category": 'Category A', "ID": 1, "Value": 12.40},
{"Category": 'Category B', "ID": 2, "Value": 30.10},
{"Category": 'Category C', "ID": 3, "Value": 100.01}
]
df = spark.createDataFrame(data)
values = [row.ID for row in df.select('ID').collect()]
default_value = str(max(values))
values_str = [str(value) for value in values]
dbutils.widgets.dropdown("ID", default_value, values_str)