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Someone help me please, 

Open templates > 1point8 > header templates > header_welcomeblock_guest

Replace this

<form method="post" action="{$mybb->settings['bburl']}/member.php">
<input name="action" type="hidden" value="do_login" />
<input name="url" type="hidden" value="" />
<input name="quick_login" type="hidden" value="1" />

With this

<form method="post" action="{$mybb->settings['bburl']}/member.php">
<input name="action" type="hidden" value="do_login" />
<input name="url" type="hidden" value="" />
<input name="quick_login" type="hidden" value="1" />
<input name="my_post_key" type="hidden" value="{$mybb->post_code}" />

Add this in your global.css

/** Modal Start **/

.blocker {
	position: fixed;
	top: 0;
	right: 0;
	bottom: 0;
	left: 0;
	width: 100%;
	height: 100%;
	overflow: auto;
	z-index: 9999;
	padding: 20px;
	box-sizing: border-box;
	background-color: rgb(0,0,0);
	background-color: rgba(0,0,0,0.75);
	text-align: center;
}

.blocker:before{
	content: "";
	display: inline-block;
	height: 100%;
	vertical-align: middle;
	margin-right: -0.05em;
}

.blocker.behind {
	background-color: transparent;
}

.modal {
	width: 400px;
	text-align: left;
	background: #fff;
	display: inline-block;
	vertical-align: middle;
	position: relative;
	z-index: 2;
	-webkit-box-sizing: border-box;
	-moz-box-sizing: border-box;
	box-sizing: border-box;
	-webkit-border-radius: 8px;
	-moz-border-radius: 8px;
	-o-border-radius: 8px;
	-ms-border-radius: 8px;
	border-radius: 8px;
	-webkit-box-shadow: 0 0 10px #000;
	-moz-box-shadow: 0 0 10px #000;
	-o-box-shadow: 0 0 10px #000;
	-ms-box-shadow: 0 0 10px #000;
	box-shadow: 0 0 10px #000;
}

.modal a.close-modal {
	position: absolute;
	top: -12.5px;
	right: -12.5px;
	display: block;
	width: 30px;
	height: 30px;
	text-indent: -9999px;
	background: url(images/close.png) no-repeat 0 0;
}

.modal-spinner {
	display: none;
	width: 64px;
	height: 64px;
	position: fixed;
	top: 50%;
	left: 50%;
	margin-right: -32px;
	margin-top: -32px;
	background: url(images/spinner_big.gif) no-repeat center center;
	-webkit-border-radius: 8px;
	-moz-border-radius: 8px;
	-o-border-radius: 8px;
	-ms-border-radius: 8px;
	border-radius: 8px;
}

/** Modal End **/