Josef,
I have this working in Skuid 10.0.x. I think you are on the same version.
The way I made it work ‘declaratively’ is to edit the XML and replace the ‘value’ with the merge syntax for {{StartDateTime}}.
<action type="updateRow" model="Event" appendorprepend="prepend" defaultmodefornewitems="edit" affectedrows="context" fieldmodel="Event" field="EndDateTime" enclosevalueinquotes="false" value="<b>{{StartDateTime}}</b>"/>
Here is my XML:
<skuidpage unsavedchangeswarning="yes" personalizationmode="server" useviewportmeta="true" showsidebar="true" showheader="true" tabtooverride="Event">
<models>
<model id="Event" limit="1" query="false" createrowifnonefound="true" datasource="salesforce" sobject="Event" type="">
<fields>
<field id="Subject"/>
<field id="EndDateTime"/>
<field id="StartDateTime"/>
</fields>
<conditions/>
<actions>
<action>
<actions>
<action type="updateRow" model="Event" appendorprepend="prepend" defaultmodefornewitems="edit" affectedrows="context" fieldmodel="Event" field="EndDateTime" enclosevalueinquotes="false" value="{{StartDateTime}}"/>
</actions>
<events>
<event>row.updated</event>
</events>
<fields>
<field>StartDateTime</field>
</fields>
</action>
</actions>
</model>
</models>
<components>
<pagetitle model="Event" uniqueid="sk-uji7t-91">
<maintitle>
<template>New {{Model.label}}</template>
</maintitle>
<subtitle>
<template>{{Model.labelPlural}}</template>
</subtitle>
<actions>
<action type="savecancel" afterCancel="/{{Model.KeyPrefix}}/o" afterSave="/{{Id}}" rollbackonanyerror="true" uniqueid="sk-uji7s-90"/>
</actions>
</pagetitle>
<basicfieldeditor showsavecancel="false" showheader="true" model="Event" mode="edit" uniqueid="sk-uji85-100">
<columns>
<column width="50%">
<sections>
<section title="Basics" collapsible="no">
<fields>
<field id="Subject" uniqueid="sk-uji7z-97"/>
<field uniqueid="sk-ujlGQ-130" id="StartDateTime" valuehalign="" type=""/>
<field uniqueid="sk-ujlGQ-129" id="EndDateTime"/>
</fields>
</section>
</sections>
</column>
<column width="50%">
<sections>
<section title="Additional Info">
<fields/>
</section>
</sections>
</column>
</columns>
</basicfieldeditor>
</components>
<resources>
<labels/>
<css/>
<javascript/>
</resources>
<styles>
<styleitem type="background" bgtype="none"/>
</styles>
</skuidpage>
Thanks,
Bill