##hiddenでuser_idを送りたい時

<%= form.hidden_field :user_id, value:current_user.id %>

new.html.erb

<%= form_with(model: post, local: true) do |form| %>
  <% if post.errors.any? %>
    <div id="error_explanation">
      <h2><%= pluralize(post.errors.count, "error") %> prohibited this post from being saved:</h2>

      <ul>
      <% post.errors.full_messages.each do |message| %>
        <li><%= message %></li>
      <% end %>
      </ul>
    </div>
  <% end %>

  <div class="field">
    <%= form.label :comment %>
    <%= form.text_field :comment %>
  </div>

  <%= form.hidden_field :user_id, value:current_user.id %>   //ここ

  <div class="actions">
    <%= form.submit %>
  </div>
<% end %>

ついでに

posts_controller.rb


  def new
    @post = Post.new
  end

  def create
    @post = Post.new(post_params)

    respond_to do |format|
      if @post.save
        format.html { redirect_to @post, notice: 'Post was successfully created.' }
        format.json { render :show, status: :created, location: @post }
      else
        format.html { render :new }
        format.json { render json: @post.errors, status: :unprocessable_entity }
      end
    end
  end

  private

    def post_params
      params.require(:post).permit(:comment, :user_id)
    end

アソシエーション

user.rb

has_many :posts

post.rb

belongs_to :user, optional: true