EUR is automatically converted to USD when you buy US stocks. When you sell US stocks, USD is automatically converted back to EUR. Manual FX conversion is not currently available. USD cannot be held in Webull EU accounts.
If you are selling U.S. stocks, the conversion from USD back to EUR occurs at the SOD (Start of Day) on T+1, and this happens only once per day. Therefore, when you sell U.S. stocks, you will not receive EUR until T+1 and will not be able to make a withdrawal until then.