Problem Definition

The customer wants to create a custom field where the users can select multiple values to the field.
Currently, there's the field type Select list (multiple choices), but it's design is too different from every other field, it does not follow the pattern of other fields. That causes it to be "not user-friendly".

Suggested Solution

It would be great to have a custom field with the same functionality as the Select list (multiple choices), but displayed like the Component/s field (with limited options of values), for example (in line with dropdown, and the ability of filtering results).

Basically, you would be able to create that custom field (Multiple choice dropdown list) and create it's possible values, just like you do when creating the Select list (multiple choices) custom field. 

It could be described as the label field with limited possible values (the ones you create) and with no possibility of creating new values (like it happens when you type a different value in the label field).

Why this is important

It would make the overall design more user friendly, for it feels that the Select list (multiple choices) field should not even be there, his design is way more distinct and occupies too much space in the screen.
Keeping patterns helps keeping the UI user-friendly.