1) Click Admin Settings
2) Click User Migration
3) Select User Migration Fields
4) Navigate to Custom Properties section
5) Turn on Custom Fields by clicking the checkbox
6) Select an unused Custom List Field, Give it a name, check to have it appear In Details and On Grid, select a Grid column for it to appear.
7) Hit Save
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That will create a column to hold the data in the grid.
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and will also create a Dropdown in the User Record as per below.
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Now that the Custom List is in place here is how you populate the list with values.
1) Click Admin Settings
2) Select Initial Setup
3) Click on Dropdown List Items
4) Click on "Click here to Add new item" a dropdown will appear allowing you to select the type of DropDown List Item you want to add.
5) Since I used Custom Property List 1 I select UserMigrationCustomPorpertyList1, once selected MigrationStudio will fill out the correct value in the Label Column, all you have to do is
6) Fill out your required string in the value column and ensure Is visible is ticked.
7) Click Save List Changes.
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Repeat Steps 4-7 for each string you wish to add to DropDown.
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Also, be aware for MigrationStudio client to pick up on changes like this you will need to refresh your browser after you make them.