Request__Share reqShr  = new Request__Share();

// Set the ID of record being shared.
reqShr.ParentId = 'a3p1w00000008OJAAY';

// Set the ID of user or group being granted access.
reqShr.UserOrGroupId = '0051w000000weEFAAY';

// Set the access level.
reqShr.AccessLevel = 'Edit';

// Set rowCause to 'manual' for manual sharing.
// This line can be omitted as 'manual' is the default value for sharing objects.
reqShr.RowCause = Schema.Request__Share.RowCause.Has_Review__c;

// Insert the sharing record and capture the save result.
// The false parameter allows for partial processing if multiple records passed
// into the operation.
Database.SaveResult sr = Database.insert(reqShr,false);

// Process the save results.
if(sr.isSuccess()){
    // Indicates success
    system.debug('success');
}
else {
    // Get first save result error.
    Database.Error err = sr.getErrors()[0];

    // Check if the error is related to trival access level.
    // Access level must be more permissive than the object's default.
    // These sharing records are not required and thus an insert exception is acceptable.
    if(err.getStatusCode() == StatusCode.FIELD_FILTER_VALIDATION_EXCEPTION  &&
            err.getMessage().contains('AccessLevel')){
    // Indicates success.
        system.debug('success');
    }
    else{
    // Indicates failure.
        system.debug(err);
    }
}