Talk to Dermatologist on Possible STI(syphilis)

3-4 weeks back, a rash appeared on my genitalia. Thought it's a fungal rash, used climatrzole for a few days. It got better now rash is recovered but area is a bit itchy. Now, i realised it appeared after an intimacy encounter. The attached picture is from 2nd week. Please suggest me any blood tests for confirmations. Or any medication in meanwhile so it doesn't get worse. There are not any symptoms like fever, fatigue, headache, flue, cough, sour throat, etc.