credit is not taxed but it matters how you pay off that credit. Say you use a credit card for all your spending in Thailand and every month you clear the balance on your credit card using money in a bank account which you have had paid in there from monthly income. Then you are in effect remitting that income and would be due tax on it. Obviously if you clear the balance in some other way not involving income that doesn't apply
You can't apply while inside Thailand. You need to travel to a country that has a Thai embassy, apply there and remain there until the process completes