diff --git a/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java b/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java
index 3a200ce..1cffcab 100644
--- a/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java
+++ b/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java
@@ -109,7 +109,7 @@ public final class AuthlibInjector {
 		ExecutionEnvironment side = detectSide();
 		Logging.LAUNCH.fine("Detected side: " + side);
 
-		apiRoot = appendSuffixSlash(apiRoot);
+		apiRoot = parseInputUrl(apiRoot);
 		Logging.CONFIG.info("API root: " + apiRoot);
 		warnIfHttp(apiRoot);
 
@@ -165,6 +165,16 @@ public final class AuthlibInjector {
 		}
 	}
 
+	private static String parseInputUrl(String url) {
+		String lowercased = url.toLowerCase();
+		if (!lowercased.startsWith("http://") && !lowercased.startsWith("https://")) {
+			url = "https://" + url;
+		}
+
+		url = appendSuffixSlash(url);
+		return url;
+	}
+
 	private static ExecutionEnvironment detectSide() {
 		String specifiedSide = System.getProperty(PROP_SIDE);
 		if (specifiedSide != null) {