adding a block of matrix field from a front end entry form
With the help of CreateSean comment I have managed to fix the problems in the form.
Here it is for future reference for everyone:
&lt;form method=&quot;post&quot; accept-charset=&quot;UTF-8&quot; enctype=&quot;multipart/form-data&quot; class=&quot;edit-store&quot;&gt;
    {{ csrfInput() }}
    {{ actionInput('entries/save-entry') }}
    {{ redirectInput('members-zone/{slug}') }}
    {{ hiddenInput('movementCoursesEntries', '2') }}
    {{ hiddenInput('enabled', '1') }}
    &lt;input type=&quot;hidden&quot; name=&quot;entryId&quot; value=&quot;{{ entry.id }}&quot;&gt;

    {# various includes here #}
    &lt;input type=&quot;hidden&quot; name=&quot;fields[blocks]&quot; /&gt;

    {% for block in entry.videos %}
        {% if block.type == 'video_box' %}
            &lt;div class=&quot;matrixblock &quot;&gt;
                &lt;input type=&quot;hidden&quot; name=&quot;fields[videos][{{ block.id }}][type]&quot; value=&quot;video_box&quot;&gt;
                &lt;input type=&quot;hidden&quot; name=&quot;fields[videos][{{ block.id }}][enabled]&quot; value=&quot;1&quot;&gt;
                &lt;input type=&quot;hidden&quot; name=&quot;fields[videos][{{ block.id }}][fields][videoTitle]&quot; value=&quot;{{ block.videoTitle }}&quot;&gt;
                &lt;input type=&quot;hidden&quot; name=&quot;fields[videos][{{ block.id }}][fields][videoUrl]&quot; value=&quot;{{ block.videoUrl }}&quot;&gt;
            &lt;/div&gt;
        {% endif %}
    {% endfor %}
    {# hidden field with block type that is being updated #}
    &lt;input type=&quot;hidden&quot; name=&quot;fields[videos][new_0][type]&quot; value=&quot;video_box&quot;&gt;
    {# is this block enabled #}
    &lt;input type=&quot;hidden&quot; name=&quot;fields[videos][new_0][enabled]&quot; value=&quot;1&quot;&gt;
    &lt;input type=&quot;text&quot; name=&quot;fields[videos][new_0][fields][videoTitle]&quot; value=&quot;&quot; placeholder=&quot;Video Title&quot;&gt;
    &lt;input type=&quot;text&quot; name=&quot;fields[videos][new_0][fields][videoUrl]&quot; value=&quot;&quot;  placeholder=&quot;Videl URL&quot;&gt;
    
    &lt;input type=&quot;submit&quot; value=&quot;Save&quot; class=&quot;btn btn-primary&quot;&gt;
&lt;/form&gt;