Add fields

assets/css/backend.css

@@ -0,0 +1,24 @@
+.it-login-button {
+    background-color: hsl(191.6, 43.4%, 55.2%);
+    box-shadow: 2px 2px 4px hsl(191.6, 43.4%, 60.2%);
+    border-radius: 8px;
+    width: max-content;
+    color: hsl(191.6, 43.4%, 10.2%);
+    display: flex;
+    height: 55px;
+    padding: 0 15px;
+    align-items: center;
+    text-decoration: none;
+    font-weight: bold;
+}
+
+.it-login-button:hover {
+    background-color: hsl(161.6, 43.4%, 55.2%);
+    color: hsl(191.6, 43.4%, 10.2%);
+}
+
+.it-login-button > img {
+    width: 40px;
+    height: 40px;
+    margin-right: 10px;
+}

templates/access-key-input.htm

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+<?php if ($value === null): ?>
+<p>
+Sie sind nicht mit Immotooler verbunden
+</p>
+<p>
+<a href="https://app.immotooler.com/oauth/authorize" class="it-login-button">
+    <img src="<?php echo $pluginDir; ?>resources/img/immotooler.png">
+    <span>Mit Immotooler verbinden</span>
+</a>
+</p>
+<?php else: ?>
+<input type="color" value="<?php echo esc_attr($value); ?>" name="it-slider-options[<?php echo esc_attr($name); ?>]">
+<?php endif; ?>