How am i supposed to add a field to the layout of a work item?

I try do add a field to a work item using this api : https://learn.microsoft.com/en-us/rest/api/azure/devops/processes/fields/add?view=azure-devops-rest-7.1&tabs=HTTP

It work (at least it did accept it, and return te good answer ) ! But the field does not appear in the layout of the work item in the UI, how am i supposed to do that ?

Thanks @Shamrai Aleksander, as mentioned by him, you need to add the custom field to one group. Besides, only inherited process supports customization. You can refer to the following PowerShell scripts.

$token = "{PAT}"
$token = [System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes(":$($token)"))
$head = @{ Authorization =" Basic $token" }
$orgname = "{Organization name}"
$processId = "{Process id of your custom process}"
$witRefName = "{The reference name of work item type}" #processname.workitemtype
$groupid = "Agile.Task.Task.Planning" #The ID of the group to add the control to.

# Add a field named 'CustomField' to work item type 'Task'
$url1="https://dev.azure.com/$orgname/_apis/work/processes/$processId/workItemTypes/$witRefName/fields?api-version=5.0-preview.2"
$body1 = @'
{  
    “referenceName”: “Custom.CustomField”,  
    “type”: “string”,  
    “readOnly”: false,  
    “required”: false,  
    “allowGroups”: null  
}  
'@
Invoke-RestMethod -Uri $url1 -Method Post -Headers $head -Body $body1 -ContentType application/json

# Add custom field 'CustomField' to 'Planning' group
$url2="https://dev.azure.com/$orgname/_apis/work/processes/$processId/workItemTypes/$witRefName/layout/groups/$groupid/controls?api-version=5.0-preview.1"

$body2 = @'
{  
    “order”:null,  
    “label”:“CustomField”,  
    “readOnly”:false,  
    “visible”:true,  
    “controlType”:null,  
    “id”:“Custom.CustomField" ,  
    “metadata”:null,  
    “inherited”:null,  
    “overridden”:null,  
    “watermark”:null,  
    “contribution”:null,  
    “height”:null,  
    “isContribution”:false  
}  
'@

Invoke-RestMethod -Uri $url2 -Method POST -Headers $head -Body $body2 -ContentType application/json

Then I can see it in Task's Planning group.