TYPO3 FlexForm - Selectbox with Multiple selection Field

 You can use below code to get Selectbox with Multiple selection field in TYPO3 Flex Form:

<label>Selectbox with Multiple selection field</label>

<config>
<type>select</type>
<items type="array">

<numindex index="0" type="array">
<numindex index="0">choice 1</numindex>
<numindex index="1">value 1</numindex>
</numindex>

<numindex index="1" type="array">
<numindex index="0">choice 2</numindex>
<numindex index="1">value 2</numindex>
</numindex>

<numindex index="3" type="array">
<numindex index="0">choice 3</numindex>
<numindex index="1">value 3</numindex>
</numindex>

</items>
<maxitems>3</maxitems>
<size>3</size>
</config>