To create a custom field for any module you should have the correct permissions:
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Step 1: Select Settings from the top info bar.
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Step 2: Select Custom Field from the list.
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Step 3: Select the module for which you are willing to add the custom field.
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Step 4: Select Add field
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A form will appear, as shown in the image below.
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Step 4: Select the Module (Client, Partner, or Product) on which you want to add the field.
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Step 5 – Name the section of your Custom field.
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Step 6 – Give the Custom Field Name.
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Step 7 – Select the Field Type (Text/Date/Number/Dropdown).
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Step 8 – Here, you have two options. Check on Make this field Mandatory to add a Mandatory field and Check on Show this field on list view to view the field on related module’s table head.
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If you select the Dropdown option, you can also select Allow Multiple Dropdown Selections so that multiple options from the dropdown can be selected.