Follow the process required to create a link to record field type to another board.

Then click on the '+' sign to the right of the last field header on the current board.

This pops up a modal or slider to display all field types (including system-generated ones) you can create on your board.

Here you can scroll through or type the Lookup field (that’s under system generated) to pick this field type.

Fill in details such as the field name and description and pick the link to record field that exists on the board.

That displays a list of other fields that exists on other board you can get records from

Select any of the fields and mark any of the checkboxes to assign field characteristics to the field.

Click on the “Create Field” button to add the field to the board.