You can use ClearTax GST software to collect GSTIN of your customers by following below steps: 1. Select the customers from whom you want to collect the GSTIN. 2. ClearTax GST software will send them an email and a SMS to the selected customers. 3. Those who have got the mail are required to fill up the form and submit the webpage. 4. The data will automatically be populated in your contact master list.