payload = {
    'username': 'some_username',
    'password': 'some_password'
}

with requests.Session() as s:
    p = s.post('https://osu.ppy.sh/forum/ucp.php?mode=login', data=payload)
    print(p.text)

<td valign="top"><b class="gensmall">Username:</b></td>
<td><input class="post" type="text" name="username" size="25" maxlength="40" value="some_username" tabindex="1"/>
</td>
</tr>
<tr>
<td valign="top"><b class="gensmall">Password:</b></td>
<td>
<input class="post" type="password" name="password" size="25" maxlength="100" tabindex="2"/>
</td>

value="some_username"

type="password"