Customer table:
id INT
name VARCHAR(32)
title_id TINYINT [FK]

Title table:
id INT
code VARCHAR(8)
name VARCHAR(24)

/**
 * Supplier
 *
 * @ORMTable(name="supplier", indexes={@ORMIndex(name="title_id", columns={"title_id"})})
 * @ORMEntity
 */
class Supplier
{
    /**
     * @var int
     *
     * @ORMColumn(name="id", type="int", nullable=false, options={"unsigned"=true})
     * @ORMId
     * @ORMGeneratedValue(strategy="IDENTITY")
     */
    private $id;

    /**
     * @var string
     *
     * @ORMColumn(name="name", type="string", length=48, nullable=false)
     */
    private $name;

    /**
     * @var Title
     *
     * @ORMManyToOne(targetEntity="Title")
     * @ORMJoinColumns({
     *   @ORMJoinColumn(name="title_id", referencedColumnName="id")
     * })
     */
    private $title;


    public function getId(): ?int
    {
        return $this->id;
    }

    public function getName(): ?string
    {
        return $this->name;
    }

    public function setName(string $name): self
    {
        $this->name = $name;

        return $this;
    }

    public function getTitle(): ?Title
    {
        return $this->title;
    }

    public function setTitle(?Title $title): self
    {
        $this->title = $title;

        return $this;
    }

    public function __toString()
    {
        return $this->name;
    }
}

/**
 * Title
 *
 * @ORMTable(name="title")
 * @ORMEntity
 */
class Title
{
    /**
     * @var bool
     *
     * @ORMColumn(name="id", type="boolean", nullable=false)
     * @ORMId
     * @ORMGeneratedValue(strategy="IDENTITY")
     */
    private $id;

    /**
     * @var string
     *
     * @ORMColumn(name="code", type="string", length=8, nullable=false)
     */
    private $code;

    /**
     * @var string
     *
     * @ORMColumn(name="name", type="string", length=24, nullable=false)
     */
    private $name;

    public function getId(): ?bool
    {
        return $this->id;
    }

    public function getCode(): ?string
    {
        return $this->code;
    }

    public function setCode(string $code): self
    {
        $this->code = $code;

        return $this;
    }

    public function getName(): ?string
    {
        return $this->name;
    }

    public function setName(string $name): self
    {
        $this->name = $name;

        return $this;
    }

    public function __toString()
    {
        return $this->name;
    }
}

class SupplierType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('name')
            ->add('title')
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => Supplier::class,
        ]);
    }
}

<select id="supplier_title" name="supplier[title]" class="form-control">
    <option value=""></option>
    <option value="1" selected="selected">TOKO</option>
    <option value="1" selected="selected">TOKO</option>
    <option value="1" selected="selected">TOKO</option>
</select>