How can I add a custom field to an admin form in Magento 2?
You can check the magento sample module they have provided
<field name="color">
<argument name="data" xsi:type="array">
<item name="config" xsi:type="array">
<!--component constructor-->
<item name="component" xsi:type="string">Magento_SampleForm/js/form/element/color-select</item>
<!--main template for form field that renders elementTmpl as a child template-->
<item name="template" xsi:type="string">ui/form/field</item>
<!--customized form element template that will show colors-->
<item name="elementTmpl" xsi:type="string">Magento_SampleForm/form/element/color-select</item>
<item name="label" xsi:type="string">Autumn colors</item>
<item name="visible" xsi:type="boolean">true</item>
<item name="dataType" xsi:type="string">text</item>
<item name="formElement" xsi:type="string">input</item>
<item name="source" xsi:type="string">sampleform</item>
</item>
</argument>
</field>
I am not sure, but I think shopping cart price rule
will give you some hint about this, here is the example
<field name="stop_rules_processing">
<argument name="data" xsi:type="array">
<item name="config" xsi:type="array">
<item name="dataType" xsi:type="string">boolean</item>
<item name="formElement" xsi:type="string">checkbox</item>
<item name="source" xsi:type="string">sales_rule</item>
<item name="prefer" xsi:type="string">toggle</item>
<item name="valueMap" xsi:type="array">
<item name="true" xsi:type="number">1</item>
<item name="false" xsi:type="number">0</item>
</item>
<item name="default" xsi:type="number">0</item>
<item name="label" xsi:type="string" translate="true">Discard subsequent rules</item>
</item>
</argument>
</field>
<container name="actions_apply_to" >
<argument name="data" xsi:type="array">
<item name="config" xsi:type="array">
<item name="sortOrder" xsi:type="number">40</item>
</item>
</argument>
<htmlContent name="html_content">
<argument name="block" xsi:type="object">Magento\SalesRule\Block\Adminhtml\Promo\Quote\Edit\Tab\Actions</argument>
</htmlContent>
</container>
For more detail you can visit this file
\vendor\magento\module-sales-rule\view\adminhtml\ui_component\sales_rule_form.xml