I've been trying to retrieve unique values by using the expression builder in QGIS 3.4

I tried the expression array_distinct(array("cab_no."))

where "cab_no." is the field who has the values.

but there is completely no result. Am I missing something here??

I'm using QGIS 3.4 attribute form to edit the widget type. I'm trying to retrieve "cab_no." field values (only unique values) from 'U_Cabinet' layer to use it in "fdt#" field in the 'Distribution2' layer. as shown in the image below

I've been trying to retrieve unique values by using the expression builder.

I tried the expression array_distinct(array("cab_no."))

where "cab_no." is the field who has the values.

but there is no result. as shown in the image below.

Am I missing something here??