Colin, is there any way to push values into fields for the new related record via the URL? I know this can be done for URLs that launch Survey123 using Field:fieldname=value but this doesn't seem to have any impact when used with the related records in field map URL.
Solved!
Never mind, I found the solution in the documentation:
Field Maps - Deploy your map (Open your map from a link) - found under "Initiate updating a feature" section
Sample of the expression used to correct curly brackets and include feature attributes, I added colors to help others see the different parts (%22 is html encoding for double quotes):
"https://fieldmaps.arcgis.app/?referenceContext=addRelatedFeature&itemID=0c900ae2a1084d27b608233921ef1a84& featureSourceURL=https://services9.arcgis.com/QjGvjfQhsHAmqfjP/arcgis/rest/ services/DamageAssessment_Apr2018_631d0895e7d3404bb78e8e04e92d896e/FeatureServer/ 1&featureID="+Replace(Replace($feature.globalid,'{',''),'}','')+"&foreignKeyField=globalid&featureAttributes={%22incidentid%22:%22527%22,%22inspector%22:%22Frank Jones%22,%22inspdate%22:1502917218285%22}"
This expression is then used as the link URL in the pop-up text box content:
<p>
<a href="{expression/expr0} ">Add Inspection</a>
</p>