In the TOSCA functions modal add a "+" button below the existing drop down lists with text "Concatenate with"
When the button is selected the user can add another tosca function property or a yaml string value.
The user can add multiple additional entries.
The resultant value will be a tosca concatenate function, for example:

{concat 
    [ get_property [node_name,property_name],
     ‘ concat string’  ,
    [ get_property [node_name,property_name],
}